Move in-flight ChannelMonitorUpdates to ChannelManager

[TheBlueMatt]

Because ChannelMonitorUpdates can be generated for a
channel which is already closed, and must still be tracked
through their completion, storing them in a Channel
doesn't make sense - we'd have to have a redundant place to
put them post-closure and handle both storage locations
equivalently.

Instead, here, we move to storing in-flight
ChannelMonitorUpdates to the ChannelManager, leaving
blocked ChannelMonitorUpdates in the Channel as they
were.

This is the first (and largest) step towards #2168, and after this we're at least clear of the serialization-breaking parts, so could in theory cut a test version even without fully fixing 2168 (which needs fixing for 116 though).

Most of the work for #2317