Re-work PackageSolvingData::absolute_tx_timelock

Re-work PackageSolvingData::absolute_tx_timelock

Previously, this would return the earliest height the output could be
confirmed, which seems to no longer be useful. The only use of the
method was to determine whether we should delay a package to a future
block. Instead, we choose to return the absolute locktime the
transaction spending the output should have, which better corresponds to
the method name and still supports the delay functionality mentioned.

Doing so also allows us to expose the locktime required for HTLC
transactions we need to broadcast based on our own commitments for
anchor channels.