Rework get_latest_holder_commitment_txn to broadcast for users

Rework get_latest_holder_commitment_txn to broadcast for users

This method is meant to be used as a last resort when a user is forced
to broadcast the current state, even if it is stale, in an attempt to
claim their funds in the channel. Previously, we'd return the commitment
and HTLC transactions such that they broadcast them themselves. Doing so
required a different code path, one which was not tested, to obtain
these transactions than our usual path when force closing. It's not
worth maintaining both, and it's much simpler for us to broadcast
instead.