mon: handle -EAGAIN in completion contexts

We can get ECANCELED, EAGAIN, or success out of the completion contexts,
but in the EAGAIN case (meaning there was an election) we were sending
a success to the client.  This resulted in client hangs and all-around
confusion when the monitor cluster was thrashing.

Backport: bobtail
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Joao Luis <joao.luis@inktank.com>
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>
Reviewed-by: Joao Luis <joao.luis@inktank.com>