When frank QUITs, people may get regreeted after he rejoins

[Merovius]

Currently, the following can happen:

a) frank shuts down
b) user who has been inactive for a while PARTs or QUITs
c) frank rejons
d) user rejons
e) user gets greeted, because they got expunged from the lastSeen list due to inactivity and frank didn't see the QUIT

As frank now keeps state over who is in which channel, we could use that information to inform greetings, instead of only refreshing the list when things get posted. We might get away with only using the member-list, but there might be corner cases I'm not thinking of. Using both, in any way, would make greetings strictly more reliable.