mon/PaxosService: prevent reads until initial service commit is done

Do not process reads (or, by PaxosService::dispatch() implication, writes)
until we have committed the initial service state.  This avoids things like
EPERM due to missing keys when we race with mon creation, triggered by
teuthology tests doing their health check after startup.

Fixes: #5515
Backport: cuttlefish
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>
(cherry picked from commit d08b6d6df7dba06dad73bdec2c945f24afc02717)