If needed, sortStorageList can be extended in the
future to support a 'readable' parameter.

We can never receive several answers from the same node.

testVerification is dropped for the same reason as for testEvent and most of
testConnection, since there is much incoming changes for verification.

- Last known TID was not updated when recovering a transaction.
- Missing OIDs were ignored, which caused partial transactions to be committed
  instead of being deleted.

The previous SQL query caused a full table scan of the 'trans' table at startup.

I used git-diff for each file and concatenated the result to preverse the order.

This fixes invalid next_serial entries in cache,
and the following error for values not in cache:

  Traceback (most recent call last):
    File "ZODB/Connection.py", line 856, in setstate
      self._setstate(obj)
    File "ZODB/Connection.py", line 894, in _setstate
      self._load_before_or_conflict(obj)
    File "ZODB/Connection.py", line 922, in _load_before_or_conflict
      if not self._setstate_noncurrent(obj):
    File "ZODB/Connection.py", line 945, in _setstate_noncurrent
      assert end is not None
  AssertionError

When run with MySQL, testBasicStore (neo.tests.threaded.test.Test) was slow
and generated log exceeded 29MB.

This increases the number of rows to check per AskCheck*Range packets.

And document 3 bugs found by running many times testBackupNodeLost. About the
tic() issue, I had a case where the test exited instead of looping forever after
the storage crash.

Before, it was only done for 'logfile'.

- Review error handling. Only 2 exceptions remain in connector.py:

  - Drop useless exception handling for EAGAIN since it should not happen
    if the kernel says the socket is ready.
  - Do not distinguish other socket errors. Just close and log in a generic way.
  - No need to raise a specific exception for EOF.
  - Make 'connect' return a boolean instead of raising an exception.
  - Raise appropriate exception when answer/ask/notify is called on a closed
    non-MT connection.

- Add support for more complex connectors, which may need to write for a read
  operation, or to read when there's pending data to send. This will be
  required for SSL support (more exactly, the handshake will be done in
  a transparent way):

  - Move write buffer to connector.
  - Make 'receive' fill the read buffer, instead of returning the read data.
  - Make 'receive' & 'send' return a boolean to switch polling for writing.
  - Tolerate that sockets return 0 as number of bytes sent.

- In testConnection, simply delete all failing tests, as announced
  in commit 71e30fb9.

Application will hold SSL parameters.

This frees a reference to the last handler and there's no need to make the
instance reusable.

This follows the behaviour of FileStorage.

There remain only one leak in ClientApplicationTests.test_connectToPrimaryNode
because of Mock objects.

admin: do not reset the list of known masters from configuration (or command line) when reconnecting

This is questionable but a lot of NodeManager must be reviewed if we want to do
differently. At least, admin nodes now behave like clients.

It's been a long time that the polling thread never ends and don't need to be
restarted. On the other side, there will be a need for the admin to define a
different polling loop, hence the move from threaded_poll to threaded_app.