mon/Paxos: adjust trimming defaults up; rename options

- trim more at a time (by an order of magnitude)
- rename fields to paxos_trim_{min,max}; only trim when there are min items
  that are trimmable, and trim at most max items at a time.
- adjust the paxos_service_trim_{min,max} values up by a factor of 2.

Since we are compacting every time we trim, adjusting these up mean less
frequent compactions and less overall work for the monitor.

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