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>
The is_readable() output *can* be useful if we have a cluster with flapping monitors, as more often than not it's due to lease expiring before they should have. I have this vague recollection that the monitor's default debug level was bumped exactly to catch the lease timeouts, as they're outputted at 1 -- and in that case the lease message is useful to match across monitors.

However, I agree that this is spamming the logs and we shouldn't see it unless we actually mean to. This should be bumped to something, but I don't think "2" is the way to go -- we seldom recall which messages are outputted on which level, and I don't ever recall asking for a level besides '1', '5' or '10' when debugging something.  If we're bumping this message, I think 5 would be best.