mon/MonitorDBStore: add get_{keys,bytes}() accounting to Transaction

Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit fd421b26748e872ddf8e0f068dda2106853edff1)
mon/Paxos: add perfcounters for most paxos operations

I'm focusing primarily on the ones that result in IO here.

Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit b09b8563d35dda23faed43afef2a983e93a879c5)