Don't persist gm_pids; by definition when all we have is the rabbit_durable_queue record the cluster has gone down completely so there can't be any pids. Fixes gm_pids leak. Since we're here, fix a similar but less severe leak in sync_slave_pids. (Less severe because rabbit_mirror_queue_misc:store_updated_slaves/1 has the effect of cleaning it up periodically. But it's still wrong.)
Add a TODO for if we are ever unbusy enough to look at removing this code.
Merged bug25638 into stable
Make it clearer what I meant.