This task is done by the admin node, in 2 possible ways:
- email notifications, as soon as some state change;
- new 'neoctl print summary' command that can be used periodically
  to check the health of the database.
They report the same information.

About backup clusters:

The admin of the main cluster also monitors selected backup clusters,
with the help of their admin nodes.

Internally, when a backup master node connects to the upstream master node,
it receives the address of the upstream admin node and forwards it to its
admin node, which is therefore able to connect to the upstream admin node.
So the 2 admin nodes remain connected and communicate in 2 ways:
- the backup node notifies upstream about the health of the backup cluster;
- the upstream node queries the backup node periodically to check whether
  replication is not too late.

TODO:

A few things are hard-coded and we may want to configure them:
- backup lateness is checked every 10 min;
- backup is expected to never be late.

There's also no delay to prevent 2 consecutive emails from having the same
Date: (unfortunately, the RFC 5322 does not allow sub-second precision),
in which case the MUA can display them in random order. This is mostly
confusing when one notification is OK and the other is not, because one
may wonder if there's a new problem.

Explicit fields in RequestIdentification are only suitable for the actual
identification or for properties that most nodes have.

But some current (and future) features require to pass values (always and
as soon as possible) for tasks that are unrelated to identification.

What Packet.setId does was overridden by Connection.answer
and that would have broken concurrent queries to the admin node
(this is something we currently don't do).

... rather than logging when the backend does not override.

Contrary to FileStorage, NEO remembers uses of readCurrent().

This fixes the following assertion:

  Traceback (most recent call last):
    File "neo/master/app.py", line 172, in run
      self._run()
    File "neo/master/app.py", line 182, in _run
      self.playPrimaryRole()
    File "neo/master/app.py", line 302, in playPrimaryRole
      self.backup_app.provideService())
    File "neo/master/backup_app.py", line 114, in provideService
      node, conn = bootstrap.getPrimaryConnection()
    File "neo/lib/bootstrap.py", line 74, in getPrimaryConnection
      poll(1)
    File "neo/lib/event.py", line 160, in poll
      to_process.process()
    File "neo/lib/connection.py", line 504, in process
      self._handlers.handle(self, self._queue.pop(0))
    File "neo/lib/connection.py", line 92, in handle
      self._handle(connection, packet)
    File "neo/lib/connection.py", line 107, in _handle
      pending[0][1].packetReceived(connection, packet)
    File "neo/lib/handler.py", line 125, in packetReceived
      self.dispatch(*args)
    File "neo/lib/handler.py", line 75, in dispatch
      method(conn, *args, **kw)
    File "neo/lib/handler.py", line 159, in notPrimaryMaster
      assert primary != self.app.server
  AttributeError: 'BackupApplication' object has no attribute 'server'

With the switch to msgpack, there was no schema anymore whereas it was
sometimes used for both automatic conversion (e.g. the last argument of
AskStoreTransaction must now be explicitly cast to list) and type checking.

This somewhat reintroduces a kind of schema that:
- is used by the test suite for type checking
- can be generated automatically from the test suite
  when one change the procotol

Not only for performance reasons (at least 3% faster) but also because of
several ugly things in the way packets were defined:
- packet field names, which are only documentary; for roots fields,
  they even just duplicate the packet names
- a lot of repetitions for packet names, and even confusion between the name
  of the packet definition and the name of the actual notify/request packet
- the need to implement field types for anything, like PByte to support new
  compression formats, since PBoolean is not enough

neo/lib/protocol.py is now much smaller.

The following 2 operations can be onerous and they should not be
directly usable without some kind of confirmation by the user:
- Dropping a node now requires to first stop it.
- Tweaking does not exclude anymore automatically DOWN nodes,
  because a node could go DOWN between the moment the user sends
  the command to tweak and the actual tweak by the master.

Initially, I wanted to do the simulation inside neoctl but it has no knowledge
of the topology (the master don't send devpath values of storage nodes).
Therefore, the work is delegated to the master node, which implies a change
of the protocol.

This stops abusing ProtocolError, which disconnects the admin node needlessly.

The many 'if ... raise RuntimeError' in neo/neoctl/neoctl.py
could be turned into assertions.

neoctl gets a new command to change the number of replicas.

The number of replicas becomes a new partition table attribute and
like the PT id, it is stored in the config table. On the other side,
the configuration value for the number of partitions is dropped,
since it can be computed from the partition table, which is
always stored in full.

The -p/-r master options now only apply at database creation.

Some implementation notes:

- The protocol is slightly optimized in that the master now sends
  automatically the whole partition tables to the admin & client
  nodes upon connection, like for storage nodes.
  This makes the protocol more consistent, and the master is the
  only remaining node requesting partition tables, during recovery.

- Some parts become tricky because app.pt can be None in more cases.
  For example, the extra condition in NodeManager.update
  (before app.pt.dropNode) was added for this is the reason.
  Or the 'loadPartitionTable' method (storage) that is not inlined
  because of unit tests.
  Overall, this commit simplifies more than it complicates.

- In the master handlers, we stop hijacking the 'connectionCompleted'
  method for tasks to be performed (often send the full partition
  table) on handler switches.

- The admin's 'bootstrapped' flag could have been removed earlier:
  race conditions can't happen since the AskNodeInformation packet
  was removed (commit d048a52d).

It is often faster to set up replicas by stopping a node (and any
underlying database server like MariaDB) and do a raw copy of the
database (e.g. with rsync). So far, it required to stop the whole
cluster and use tools like 'mysql' or sqlite3' to edit:
- the 'pt' table in databases,
- the 'config.nid' values of the new nodes.

With this new option, if you already have 1 replica, you can set up
new replicas with such fast raw copy, and without interruption of
service. Obviously, this implies less redundancy during the operation.

--kill-mysqld should be combined with something like -f .3 -r .1
to give storage nodes enough time to recover.
And also -D 0 to focus testing on the storage backend rather than NEO.

This also reverts commit 442bb43a.

This fixes up commit be839e92.