mon/Paxos: not readable when LOCKED

If we are re-proposing a previously accepted value from a previous quorum,
we should not consider it readable, because it is possible it was exposed
to clients as committed (2/3 accepted) but not recored to be committed, and
we do not want to expose old state as readable when new state was
previously readable.

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