mon: enable 'mon compact on trim' by default; trim in larger increments

This resolves the leveldb growth-without-bound problem observed by
mikedawson, and all the badness that stems from it.  Enable this by
default until we figure out why leveldb is not behaving better.

While we are at it, trim more states at a time.  This will make
compaction less frequent, which should help given that there is some
overhead unrelated to the amount of deleted data.

Fixes: #4815
Signed-off-by: Sage Weil <sage@inktank.com>