Prune full channels if either direction is not updating

[TheBlueMatt]

ie implement the change at lightning/bolts#1014 we also need to keep track of channels we removed so we don't immediately re-add them.

[naumenkogs]

I was not sure if i should take good first issues, but anyway decided to work on it and planning to open a PR in a day or two :)

[TheBlueMatt]

Awesome! Probably makes sense to base this off of #1649 just to avoid conflicts, or at least make sure its not going to conflict too heavily there, though I don't think we want to mark such channels as perm-failed.