Implement `PartialEq` for `ChannelMonitor`

Implement `PartialEq` for `ChannelMonitor`

It turns out `#[derive(PartialEq)]` will automatically bound the
`PartialEq` implementation by any bounds on the struct also being
`PartialEq`. This means to use an auto-derived `ChannelMonitor`
`PartialEq` the `EcdsaSigner` used must also be `PartialEq`, but
for the use-cases we have today for a `ChannelMonitor` `PartialEq`
it doesn't really matter - we use it internally in tests and
downstream users wanted similar test-only usage.

Fixes #1912.