mon: retry PGStats message on EAGAIN

If we get EAGAIN from a paxos restart/election/whatever, we should
restart the message instead of just blindly acking it.

Signed-off-by: Sage Weil <sage@inktank.com>
mon: assert valid context return values

We recognized EAGAIN, ECANCELED, and success only.

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