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Persistent sessionlog

ITS#8486 suggests we use a more efficient structure to maintain the
sessionlog in. If we're messing with sessionlog already, we might as
well see if we can address another issue - it is always empty on slapd
startup leading to unnecessary full refreshes happening.

slapo-accesslog has most of the data we need to support that and is
already sorted in CSN order (much like sessionlog).

AFAIK, we can't use the accesslog database directly as the database as
we can't efficiently search on a single serverID to get the serverID
set and the oldest CSN for each.

There are a few tasks that need to be done in order to achieve this:
- configure syncprov with a suffix that contains the slapo-accesslog
  style logs for our DB
- change struct sessionlog to use a more efficient structure that can be
  iterated over from any point (only tavl is available at the moment)
- on startup:
  - iterate through the *last* N entries (filtering on successful write
    ops that affect our suffix) and build slog_entry for each of those
  - for each entry, insert a new slog_entry and update sl_mincsn
- add a control to hint the database that we require the database to
  iterate from the end backward (back-[mhb]db can support this)
- update accesslog to log entryUUID for the entry that has just been
  written
- update the test suite to exercise the new failure conditions

There are some caveats to this still:
- if we aren't guaranteed to receive the accesslog entries in reverse
  CSN order, the resulting sessionlog would be quite unsafe to use, we
  have to try and detect this and start with an empty sessionlog
  instead, resetting sl_mincsn set to match the database contextCSN
- We might find an accesslog entry we can't use (modification that doesn't
  have enough information), we should still be able to use whatever we
  built until then, but can't continue

-- 
Ondřej Kuzník
Senior Software Engineer
Symas Corporation                       http://www.symas.com
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