mon: set threshold to periodically stash_full

Set an interval to periodically write a full copy of the map that is lower
than the trim point (which is generally a very large number of commits).

Signed-off-by: Sage Weil <sage@inktank.com>
mon: commit LogSummary on every message

This moves our version pointer up so that we don't re-log (by re-consuming)
log messages to /var/log/ceph/ceph.log on ceph-mon restart.  OTOH, it means
we rewrite the summary of the last 50 messages, but we consider that to be
relatively cheap (and something we *always* did prior for bobtail and
earlier anyway).

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #234 from ceph/wip-4758

Fixes #4758.

Reviewed-by: Greg Farnum <greg@inktank.com>