mon/Paxos: don't spam log with is_readable at dout level 1

Backport: firefly, dumpling
Reported-by: Aanchal Agrawal <Aanchal.Agrawal@sandisk.com>
Signed-off-by: Sage Weil <sage@redhat.com>
mon/Paxos: make is_readable debug output show result

Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2421 from ceph/wip-paxos-dout

mon/Paxos: don't spam log with is_readable at dout level 1

Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>