* master:
  client: fix item eviction from cache, which could break loading from storage
  Bump protocol version for new read-only mode in BACKUPING state
  backup: Teach cluster in BACKUPING state to also serve regular ZODB clients in read-only mode
  tests/threaded: Add handy shortcuts to NEOCluster to concisely check cluster properties in tests

`ClientCache._oid_dict` shall not have empty values. For a given oid, when the
last item is removed from the cache, the oid must be removed as well to free
memory. In some cases, this was not done.

A consequence of this bug is the following exception:

    ERROR ZODB.Connection Couldn't load state for 0x02d1e1e4
    Traceback (most recent call last):
      File "ZODB/Connection.py", line 860, in setstate
        self._setstate(obj)
      File "ZODB/Connection.py", line 901, in _setstate
        p, serial = self._storage.load(obj._p_oid, '')
      File "neo/client/Storage.py", line 82, in load
        return self.app.load(oid)[:2]
      File "neo/client/app.py", line 358, in load
        self._cache.store(oid, data, tid, next_tid)
      File "neo/client/cache.py", line 228, in store
        prev = item_list[-1]
    IndexError: list index out of range

A backup cluster for tids <= backup_tid has all data to provide regular
read-only ZODB service. Having regular ZODB access to the data can be
handy e.g. for externally verifying data for consistency between
main and backup clusters. Peeking around without disturbing main
cluster might be also useful sometimes.

In this patch:

- master & storage nodes are taught:

    * to instantiate read-only or regular client service handler depending on cluster state:
      RUNNING   -> regular
      BACKINGUP -> read-only

    * in read-only client handler:
      + to reject write-related operations
      + to provide read operations but adjust semantic as last_tid in the database
        would be = backup_tid

- new READ_ONLY_ACCESS protocol error code is introduced so that client can
  raise POSException.ReadOnlyError upon receiving it.

I have not implemented back-channel for invalidations in read-only mode (yet ?).
This way once a client connects to cluster in backup state, it won't see
new data fetched by backup cluster from upstream after client connected.

The reasons invalidations are not implemented is that for now (imho)
there is no off-hand ready infrastructure to get updates from
replicating node on transaction-by-transaction basis (it currently only
notifies when whole batch is done). For consistency verification (main
reason for this patch) we also don't need invalidations to work, as in
that task we always connect afresh to backup. So I simply only put
relevant TODOs about invalidations for now.

The patch is not very polished but should work.

/reviewed-on !4

Upon #storage reaching #autostart, with autostart being on, by
definition if recovery / verification phases can be completed, the
cluster should change its state to RUNNING.

The corresponding test (introduced in 58774fb6 "master: new option to
automatically start a new cluster") was not checking that.

* origin/master:
  neoctl: make 'print ids' command display time of TIDs
  mysql: force _getNextTID() to use appropriate/whole index

Currently, the command "neoctl [arguments] print ids" has the following output:

    last_oid = 0x...
    last_tid = 0x...
    last_ptid = ...

or

    backup_tid = 0x...
    last_tid = 0x...
    last_ptid = ...

depending on whether the cluster is in normal or backup mode.

This is extremely unreadable since the admin is often interested in the time that corresponds to each tid. Now the output is:

    last_oid = 0x...
    last_tid = 0x... (yyyy-mm-dd hh:mm:ss.ssssss)
    last_ptid = ...

or

    backup_tid = 0x... (yyyy-mm-dd hh:mm:ss.ssssss)
    last_tid = 0x... (yyyy-mm-dd hh:mm:ss.ssssss)
    last_ptid = ...

/reviewed-on !2

Similarly to 13911ca3 on the same instance after MariaDB was upgraded to
10.1.17 the following query, even after `OPTIMIZE TABLE obj`, started to execute
very slowly:

    MariaDB [(none)]> SELECT tid FROM neo1.obj WHERE `partition`=5 AND oid=79613 AND tid>268707071353462798 ORDER BY tid LIMIT 1;
    +--------------------+
    | tid                |
    +--------------------+
    | 268707072758797063 |
    +--------------------+
    1 row in set (4.82 sec)

Both explain and analyze says the query will/is using `partition` key but only partially (note key_len is only 10, not 18):

    MariaDB [(none)]> SHOW INDEX FROM neo1.obj;
    +-------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
    | Table | Non_unique | Key_name  | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
    +-------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
    | obj   |          0 | PRIMARY   |            1 | partition   | A         |    28755928 |     NULL | NULL   |      | BTREE      |         |               |
    | obj   |          0 | PRIMARY   |            2 | tid         | A         |    28755928 |     NULL | NULL   |      | BTREE      |         |               |
    | obj   |          0 | PRIMARY   |            3 | oid         | A         |    28755928 |     NULL | NULL   |      | BTREE      |         |               |
    | obj   |          0 | partition |            1 | partition   | A         |    28755928 |     NULL | NULL   |      | BTREE      |         |               |
    | obj   |          0 | partition |            2 | oid         | A         |    28755928 |     NULL | NULL   |      | BTREE      |         |               |
    | obj   |          0 | partition |            3 | tid         | A         |    28755928 |     NULL | NULL   |      | BTREE      |         |               |
    | obj   |          1 | data_id   |            1 | data_id     | A         |    28755928 |     NULL | NULL   | YES  | BTREE      |         |               |
    +-------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
    7 rows in set (0.00 sec)

    MariaDB [(none)]> explain SELECT tid FROM neo1.obj WHERE `partition`=5 AND oid=79613 AND tid>268707071353462798 ORDER BY tid LIMIT 1;
    +------+-------------+-------+------+-------------------+-----------+---------+-------------+------+--------------------------+
    | id   | select_type | table | type | possible_keys     | key       | key_len | ref         | rows | Extra                    |
    +------+-------------+-------+------+-------------------+-----------+---------+-------------+------+--------------------------+
    |    1 | SIMPLE      | obj   | ref  | PRIMARY,partition | partition | 10      | const,const |    2 | Using where; Using index |
    +------+-------------+-------+------+-------------------+-----------+---------+-------------+------+--------------------------+
    1 row in set (0.00 sec)

    MariaDB [(none)]> analyze SELECT tid FROM neo1.obj WHERE `partition`=5 AND oid=79613 AND tid>268707071353462798 ORDER BY tid LIMIT 1;
    +------+-------------+-------+------+-------------------+-----------+---------+-------------+------+------------+----------+------------+--------------------------+
    | id   | select_type | table | type | possible_keys     | key       | key_len | ref         | rows | r_rows     | filtered | r_filtered | Extra                    |
    +------+-------------+-------+------+-------------------+-----------+---------+-------------+------+------------+----------+------------+--------------------------+
    |    1 | SIMPLE      | obj   | ref  | PRIMARY,partition | partition | 10      | const,const |    2 | 9741121.00 |   100.00 |       0.00 | Using where; Using index |
    +------+-------------+-------+------+-------------------+-----------+---------+-------------+------+------------+----------+------------+--------------------------+
    1 row in set (4.93 sec)

By explicitly forcing (partition, oid, tid) index usage which is precisely designed to serve this and similar queries can avoid the query from being slow:

    MariaDB [(none)]> analyze SELECT tid FROM neo1.obj FORCE INDEX(`partition`) WHERE `partition`=5 AND oid=79613 AND tid>268707071353462798 ORDER BY tid LIMIT 1;
    +------+-------------+-------+-------+---------------+-----------+---------+------+------+--------+----------+------------+--------------------------+
    | id   | select_type | table | type  | possible_keys | key       | key_len | ref  | rows | r_rows | filtered | r_filtered | Extra                    |
    +------+-------------+-------+-------+---------------+-----------+---------+------+------+--------+----------+------------+--------------------------+
    |    1 | SIMPLE      | obj   | range | partition     | partition | 18      | NULL |    2 |   1.00 |   100.00 |     100.00 | Using where; Using index |
    +------+-------------+-------+-------+---------------+-----------+---------+------+------+--------+----------+------------+--------------------------+
    1 row in set (0.00 sec)

/cc @jm, @vpelltier, @Tyagov

/reviewed-on !1