Use channel ID over funding outpoint to track monitors in ChannelManager

As motivated by the previous commit, we do some of the same work here at
the `ChannelManager` level instead. Unfortunately, we still need to
track the funding outpoint to support downgrades by writing the in
flight monitor updates as two separate TLVs, one using the channel IDs,
and the other using the funding outpoints. Once we are willing to stop
supporting downgrades past this version, we can fully drop it.

Author: wpaulino