Move channel -> peer tracking to `OutPoint`s from Channel IDs

Move channel -> peer tracking to `OutPoint`s from Channel IDs

For backwards compatibility reasons, we need to track a mapping
from funding outpoints to channel ids. To reduce diff, this was
previously done with channel IDs, converting the `OutPoint`s to
channel IDs before using the map.

This worked fine, but is somewhat brittle - because we allow
redundant channel IDs across different peers, we had to avoid
insertion until we had a real channel ID, and thus also had to be
careful to avoid removal unless we were using a real channel ID,
rather than a temporary one.

This brittleness actually crept in to handling of errors in funding
acceptance, allowing a remote party to get us to remove an entry by
sending a overlapping temporary channel ID with a separate real
channel ID.

Luckily, this map is relatively infrequently used, only used in the
case we see a monitor update completion from a rather ancient
monitor which is unaware of the counterparty node.

Even after this change, the channel -> peer tracking storage is
still somewhat brittle, as we rely on entries not being added until
we are confident no conflicting `OutPoint`s have been used across
channels, and similarly not removing unless that check has
completed.