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