Track an EventCompletionAction for after an Event is processed

This will allow us to block `ChannelMonitorUpdate`s on `Event`
processing in the next commit.

Note that this gets dangerously close to breaking forwards
compatibility - if we have an `Event` with an
`EventCompletionAction` tied to it, we persist a new, even, TLV in
the `ChannelManager`. Hopefully this should be uncommon, as it
implies an `Event` was delayed until after a full round-trip to a
peer.

Author: TheBlueMatt