mon: compact leveldb on bootstrap

This is an opportunistic time to optimize our local data since we are
out of quorum.  It serves as a safety net for cases where leveldb's
automatic compaction doesn't work quite right and lets things get out
of hand.

Anecdotally we have seen stores in excess of 30GB compact down to a few
hundred KB.  And a 9GB store compact down to 900MB in only 1 minute.

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