Lean on the holding cell when batch-forwarding/failing HTLCs

Lean on the holding cell when batch-forwarding/failing HTLCs

When we batch HTLC updates, we currently do the explicit queueing
plus the commitment generation in the `ChannelManager`. This is a
bit strange as its ultimately really a `Channel` responsibility to
generate commitments at the correct time, with the abstraction
leaking into `ChannelManager` with the `send_htlc` and
`get_update_fail_htlc` method docs having clear comments about
how `send_commitment` MUST be called prior to calling other
`Channel` methods.

Luckily `Channel` already has an update queue - the holding cell.
Thus, we can trivially rewrite the batch update logic as inserting
the desired updates into the holding cell and then asking all
channels to clear their holding cells.