Rename tor-memtrack to tor-memquota

I found a reference to memquota in memory-limit.md and I think it might be a nicer name. To my mind, "quota" seems to have more content to it, compared to "tracker" with is rather blander. Much of the code is to do with the quota, and reclamation, rather than just accounting.

Doing a rename now is quite un-disruptive since we haven't even published tor-memtrack yet, and I think I'm probably the only person working here atm. So we should decide now, one way or the other.

I spoke to @nickm on irc and he said he's OK with either name.