Add minor-mode to run sync-watchdog timer

I don't have much info about this, but sometimes ement simply stops updating rooms on its own.

Today I've been using ement.el for many hours, and I'm running the stand-alone Element client as well (with a different auth token), and I just noticed that Element had a message in a room which ement.el wasn't showing me. As soon as I typed `g` in that room, ement.el fetched the missing message (which had been sent more than an hour earlier) as well as a bunch of others (my desktop notifications showed me a heap of messages from other rooms in rapid succession).

All I could find in the messages buffer was `ement-room-sync: Wrong type argument: arrayp, nil` which I've noticed happens when typing `g` in the Notifications or Mentions buffers. I don't know whether that could have made the auto-sync fail? (Maybe if the timer only runs upon successful completion of the previous attempt?)

I've also seen the sync failing overnight, such that I've taken to typing `g` in some room when I start using my laptop in the morning to ensure that the sync is happening.

Maybe some kind of fail-safe system could be triggered periodically to make sure that the sync system gets restarted after a while if necessary?