osd: fix divergent backfill targets

osd: fix divergent backfill targets

During peering, a previous backfill target may have a slightly newer
last_update than the other options, but it will not be chosen because it
is incomplete.  That caused a failed assert during activate() (#1983).

To fix, we remove the bad assert, and then fix merge_log() so that the
replica/backfill target will trim its divergent entries when it gets the
activation MLogRec.  We also fix the handling of MInfoRec, as that can
trigger the same analogous condition.

Signed-off-by: Sage Weil <sage.weil@dreamhost.com>