When a channel is closed, we can forget the data from historical HTLCs
sent and received through that channel (which is otherwise required to
punish cheating attempts by our peer).

We previously synchronously removed that data from the DB when the closing
transaction confirmed. However, this could create performance issues as
the `htlc_infos` table can be very large for busy nodes and many concurrent
writes may be happening at the same time.

We don't need to get rid of this data immediately: we only want to remove
it to avoid degrading the performance of active channels that read and
write to the `htlc_infos` table. We now mark channels as closed in a
dedicated table, and run a background actor that deletes batches of
obsolete htlc data at regular intervals. This ensures that the table is
eventually cleaned up, without impacting the performance of active
channels.

Fixes #2610 and #2702

When a splice transaction confirms, all the revoked commitment transactions
that only applied to the previous funding transaction cannot be published
anymore, because the previous funding output has already been spent.

We can thus forget all the historical HTLCs that were included in those
commitments, because we will never need to generate the corresponding
penalty transactions.

This ensures that the growth of our DB is bounded, and will shrink every
time a splice transaction is confirmed.

Fixes #2740

- use `forgetHtlcInfo` to avoid code duplication
- test exact number of DB entries cleaned up
- use commitment index mismatch in splice test

While it requires more duplication in the codecs, because we didn't
version the commitments codec, it is cleaner to have this filed in
the commitment instead of having it in the funding status.