Ensure `ChannelMonitor` `Logger`s are always wrapped with metadata

Ensure `ChannelMonitor` `Logger`s are always wrapped with metadata

In order to ensure log lines generated by `ChannelMonitor` always
have a counterparty and channel ID entry, this consistently wraps
`Logger`s in a decorator in all `pub(X)` `ChannelMonitor` functions,
removing `pub` markings on `ChannelMonitorImpl` methods that aren't
actually publicly reachable anyway.

This also lets us clean up the `Logger` types in various
`ChannelMonitor` methods.