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

Signed-off-by: Sage Weil <sage@redhat.com>
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>
Merge pull request #2237 from ceph/wip-8560

mon: instrument paxos

Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>