mon: enable leveldb cache by default

512 MB sounds reasonable to me.

Signed-off-by: Sage Weil <sage@inktank.com>
mon/Paxos: never write first_committed except during trim

The trimming is handled by proposing transactions.  Do not confuse matters
by writing (incorrect) first_committed values at any other point.

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