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>
mon: discard messages from disconnected clients

If the client is not connected, discard the message.  They will
reconnect and resend anyway, so there is no point in processing it
twice (now and later).

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