This fixes a regression in commit 83fe64bf
when ttrans has several rows to the same data_id.

- Server connections can now be in 'connecting' state.
- connectionAccepted event (which has never been used so far) is merged into
  connectionCompleted.

This is a workaround to fix holes if replication is interrupted after new data
is committed.

AssertionError are certainly more severe that any other exception
(including OperationFailure) because the process is in an unknown state.

This could have been useful in testStorageFailureDuringTpcFinish:
close() could not be called from answerTransactionFinished because it
deadlocked while trying to send notifications.

The test was relying on fact on the fact that 'c.abort()' caused an assertion
failure, which closed the connection and then raised OperationFailure.
Actually, I wanted to close the connection on master, but it's clearer this way.

Previous code relied on the fact that the exception target is kept past
the end of the except clause. 2to3 is not smart enough to detect that.

Without this change, a different OperationalError exception would be
ignored because there's already a local variable of the same name.

This was forgotten when this table was introduced in
commit f9a8500d

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.