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>
(cherry picked from commit 62ca27d0b119b597ebad40dde64c4d86599e466d)