mon/Paxos: be more vocal about calling elections

Signed-off-by: Sage Weil <sage@inktank.com>
mon: drop useless sync_reset() calls

These are both right before bootstrap(), which also resets.

Signed-off-by: Sage Weil <sage@inktank.com>
mon: allow others to sync from us across bootstrap calls

If someone is syncing from us and there is an election, they currently get
reset and have to restart their sync.  This can lead to situations where
they can never finish, e.g., when the load from them syncing makes us time
out commits and call elections.

There is nothing that changes during bootstrap that would prevent a sync
from proceeding.  The only time we need to stop providing is when we
ourselves decide to sync from someone else; modify that reset call to
reset provider state.  All other resets become requester resets.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge remote-tracking branch 'gh/wip-mon' into next

Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>