Use channel ID over funding outpoint to track monitors in ChainMonitor

Once dual funding/splicing is supported, channels will no longer
maintain their original funding outpoint. Ideally, we identify
`ChannelMonitor`s with a stable identifier, such as the channel ID,
which is fixed throughout the channel's lifetime.

This commit replaces the use of funding outpoints as the key to the
monitor index with the channel ID. Note that this is strictly done to
the in-memory state; it does not change how we track monitors within
their persisted state, they are still keyed by their funding outpoint
there. Addressing that is left for follow-up work.

Author: wpaulino