mon/PaxosService: trim periodically instead of via propose_pending

We want to trim old states even if there is no update activity.  For
example, if a long-running rebalance finishes all osdmap updates will
stop and we won't trim out old maps to free space.

Instead, trim at the same time as tick().  Remove the trim during
propose_pending() to force all trims through this path and avoid
introducing a new and rarely-exercised behavior.

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