Rework Aborted and Closed states

[t-bast]

It's a bit messy because we use Aborted in places where it should actually be Closed, but we can't because it requires an inner Closing.

An area to explore would be to remove Aborted and Closed entirely, and create a new action ChannelClosed that would:

• if we had a persisted state, mark it as closed in the DB and remove from _channels
• otherwise, just remove from _channels

[t-bast]

We should also rework the trait hierarchy for channel states, so that Closing cannot be wrapped in Offline or Syncing (see #539).