Replace a per-call cost with a constant one.
Signed-off-by: Julien Muchembled <jm@nexedi.com>

loadBefore is more likely than other loads, as it is how repeatable-read is
implemented and the vast majority of accesses are scoped to a transaction.
So put it first in 3-branches "if" choosing between load/loadBefore/loadSerial
code paths.
next_serial is likely to not be found, because application will likely
operate on up-to-date objects. So a test is more efficient than a
try..except. Inline this test in return statement to avoid defining a local.

Next version is actually a metapackage depending on:
  persistent 4.0
  BTrees 4.0
  ZODB 4.0
  ZEO 4.0

Note that version '123dev' is considered lower than '123'

Update (only) user.

This fixes a bug causing a crash during tpc_finish phase when a storage
involved in a transaction does not receive any object, but receives at
least one CheckCurrentSerial request: no transaction was registered, and
storage would fail to lock transaction when requested by master during
tpc_finish phase.

In particular, do not flush all debug logs still in memory