mon: set leader commands prior to first election

If we have just started and receive a command, we currently will reply with
EINVAL because the leader commands are empty.  Note that this race is very
difficult to reach because the (old) peon needs to forward a command to
the mon while it still thinks it has quorum, and the message needs to get
sent after the leader mon has restarted and reset its connection but before
it has declared a new election.

To fix this, we should assume at startup time that our commands are
valid.  If it is an internal command that does not require quorum, that
is fine.  If it does require quorum, we will retry the command after the
election completes and we will revalidate the command then.

Fixes: #8132
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1685 from ceph/wip-8132

mon: set leader commands prior to first election

Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>