For existing DB, altering the table may be doable with schema editing and
clean up of sqlite_sequence.

getObject becomes faster because it does not use secondary index anymore.
Only the primary one. This frees RAM during normal operation. For MySQL,
DatabaseManager._getObject is sped up by ~3% for in-memory loads.
An improvement of ~1% from ERP5 was also mesured for IO-bound loads.

On insertion, the fast index is (`partition`, tid, oid) because we almost
always insert lines with increasing tid, whereas oid values are more random.
Although the value (data_id+value_tid) is moved from the fast to the slow index,
this should have little impact on performance because the value size is quite
small compared to the key.

The impact on replication should also be negligible:
- a little faster when there's no oid to replicate: only the secondary index,
  smaller, is scanned
- otherwise: the (slightly) biggest index is scanned randomly

On disk usage, an increase of ~4% was observed for TokuDB.
Less compressibility ? Any link with https://jira.percona.com/browse/TDB-86 ?

This information is handy to see how well cache performs.

Amended by Julien Muchembled:
- do not abbreviate some existing field names in repr result (asking the
  user to look at the source code in order to decipher logs is not nice)
- hit: change from %.1f to %.3g
- hit: hide it completely if nload is 0
- use __future__.division instead of adding more casts to float

INFO Z2 Log files reopened successfully
INFO SignalHandler Caught signal SIGTERM
INFO Z2 Shutting down fast
INFO ZServer closing HTTP to new connections
ERROR ZODB.Connection Couldn't load state for BTrees.LOBTree.LOBucket 0xc12e29
Traceback (most recent call last):
  File "ZODB/Connection.py", line 909, in setstate
    self._setstate(obj, oid)
  File "ZODB/Connection.py", line 953, in _setstate
    p, serial = self._storage.load(oid, '')
  File "neo/client/Storage.py", line 81, in load
    return self.app.load(oid)[:2]
  File "neo/client/app.py", line 355, in load
    data, tid, next_tid, _ = self._loadFromStorage(oid, tid, before_tid)
  File "neo/client/app.py", line 387, in _loadFromStorage
    askStorage)
  File "neo/client/app.py", line 297, in _askStorageForRead
    self.sync()
  File "neo/client/app.py", line 898, in sync
    self._askPrimary(Packets.Ping())
  File "neo/client/app.py", line 163, in _askPrimary
    return self._ask(self._getMasterConnection(), packet,
  File "neo/client/app.py", line 177, in _getMasterConnection
    result = self.master_conn = self._connectToPrimaryNode()
  File "neo/client/app.py", line 202, in _connectToPrimaryNode
    index = (index + 1) % len(master_list)
ZeroDivisionError: integer division or modulo by zero

It's not possible yet to replicate a node that is importing data.
One must wait that the migration is finished.

This reverts commit d3c22487 partially
and fixes the bug in a much simpler way.

Traceback (most recent call last):
  [...]
  File "neo/storage/handlers/client.py", line 115, in askStoreObject
    *e.args)
  File "neo/lib/handler.py", line 333, in queueEvent
    self.sortQueuedEvents()
  File "neo/lib/handler.py", line 326, in <lambda>
    self._event_queue.sort(key=key))()
  File "neo/storage/transactions.py", line 67, in __lt__
    return self.locking_tid < other.locking_tid
AttributeError: 'NoneType' object has no attribute 'locking_tid'

Pending events:
  (None, <askFetchTransactions: ...>)
  (<Transaction(C13, locking_tid=03c266508a058388, tid=None, age=0.21s) at 0x7f086bbc3d50>, <_askStoreObject: ...>)

It was out-of-date since commit 23b6a66a.

This time we had issues with _getObject and getReplicationObjectList
on production.
It seems related to the upgrade of MariaDB, from 10.0.21 to 10.1.25.

See also commits eaa00a88
             and 13911ca3

The explanation became wrong during a git-rebase, when it was decided
to keep the old code that drop partitions. The new one needs more work
and it is kept in a branch.