This requires ZODB >= 5.6.0

This fixes the bug that with only email notification, monitoring
stopped checking whether backup clusters are lagging after status is
unchanged since the last check (about lagging, what is compared is
the set of lagging backups). Until another event wakes up monitoring.

The code is also simplified in that there's no need for the moment to
have a different timeout between the normal case and a smtp failure.

When concurrent transactions fail with different storages (e.g. only network
issues between C1-S2 and C2-S1), in such a way that each transaction can be
committed but not both (or the cluster would be non-operational), and if the
first transaction is aborted (between tpc_vote and tpc_finish), then the second
wrongly failed with INCOMPLETE_TRANSACTION.

And if both transactions could be committed (e.g. more than 1 replica),
some nodes would be disconnected for nothing.

This fixes:

  Traceback (most recent call last):
    ...
    File "neo/admin/handler.py", line 200, in answerLastTransaction
      app.maybeNotify(name)
    File "neo/admin/app.py", line 380, in maybeNotify
      self._notify(False)
    File "neo/admin/app.py", line 302, in _notify
      body += '', name, '    ' + backup.formatSummary(upstream)[1]
    File "neo/admin/app.py", line 74, in formatSummary
      tid = self.backup_tid if backup else self.ltid
  AttributeError: 'Backup' object has no attribute 'backup_tid'

This fixes:

  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 314, in playPrimaryRole
      self.backup_app.provideService())
    File "neo/master/backup_app.py", line 101, in provideService
      app.changeClusterState(ClusterStates.STARTING_BACKUP)
    File "neo/master/app.py", line 474, in changeClusterState
      ) or not node.isClient(), (state, node)
  AssertionError: (<EnumItem STARTING_BACKUP (4)>, <ClientNode(uuid=C1, state=RUNNING, connection=<ServerConnection(nid=C1, address=127.0.0.1:52430, handler=ClientReadOnlyServiceHandler, fd=59, on_close=onConnectionClosed, server) at 7f38f5628390>) at 7f38f5628ad0>)

In such case, it didn't reconnect, but thought it was connected,
which eventually led to crashes like:

  Traceback (most recent call last):
    ...
    File "neo/admin/handler.py", line 130, in answerClusterState
      self.app.updateMonitorInformation(None, cluster_state=state)
    File "neo/admin/app.py", line 274, in updateMonitorInformation
      self.upstream_admin_conn.send(Packets.NotifyMonitorInformation(kw))
    File "neo/lib/connection.py", line 565, in send
      raise ConnectionClosed
  neo.lib.connection.ConnectionClosed

This fixes:

  Traceback (most recent call last):
    File "neo/scripts/neoadmin.py", line 31, in main
      app.run()
    File "neo/admin/app.py", line 179, in run
      self._run()
    File "neo/admin/app.py", line 199, in _run
      self.em.poll(1)
    File "neo/lib/event.py", line 155, in poll
      self._poll(blocking)
    File "neo/lib/event.py", line 220, in _poll
      if conn.readable():
    File "neo/lib/connection.py", line 487, in readable
      self._closure()
    File "neo/lib/connection.py", line 545, in _closure
      self.close()
    File "neo/lib/connection.py", line 534, in close
      handler.connectionFailed(self)
    File "neo/admin/handler.py", line 210, in connectionClosed
      app.connectToUpstreamAdmin()
    File "neo/admin/app.py", line 230, in connectToUpstreamAdmin
      None, None, self.name, None, {}))
    File "neo/lib/connection.py", line 574, in ask
      raise ConnectionClosed
  neo.lib.connection.ConnectionClosed

- make the stress process log to stress.log
- log decisions to firewall/kill nodes
- new --backlog option

Stress code reuses the admin application class and the latter
was changed in commit e434c253.

Same as commit a00ab78b.

It was reverted mistakenly when switching to msgpack.

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().