Introduce FundingScope abstraction to ChannelMonitor

When establishing a channel, the funding transaction may be replaced
either:

- after the funding transaction has confirmed using splicing,
- before the funding transaction has confirmed for v2 channel
  establishment using `tx_init_rbf`, or
- before the splice's funding transaction has confirmed using
  `tx_init_rbf`.

In each of these cases, fields in the `ChannelMonitor` will need to be
updated once the new funding transaction confirms. This commit
introduces a `FundingScope` to hold the aforementioned fields, allowing
to swap in another `FundingScope` when necessary.

This is the same abstraction as in `channel.rs` representing a different
set of data.

Author: wpaulino