mon: simplify states

- make states mutually exclusive (an enum)
- rename locked -> updating_previous
- set state prior to begin() to simplify things a bit

Signed-off-by: Sage Weil <sage@inktank.com>
mon/PaxosService: not active during paxos UPDATING_PREVIOUS

Treat this as an extension of the recovery process, e.g.

 RECOVERING -> ACTIVE
or
 RECOVERING -> UPDATING_PREVIOUS -> ACTIVE

and we are not active until we get to "the end" in both cases.

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