mon/PaxosService: simplify readable check

Recast this in terms of the paxos check and our additional conditions,
which match wait_for_readable().

Signed-off-by: Sage Weil <sage@inktank.com>
mon/PaxosService: simplify is_writeable

Recast this in terms of paxos check + our conditions, and make it
match wait_for_writeable().

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