Rename `soonest_conf_deadline` to `counterparty_spendable_height`

Rename `soonest_conf_deadline` to `counterparty_spendable_height`

This renames the field in `PackageTemplate` which describes the
height at which a counterparty can make a claim to an output to
match its actual use.

Previously it had been set based on when a counterparty can claim
an output but also used for other purposes. In the previous commit
we cleaned up its use for fee-bumping-rate, so here we can rename
it as it is now only used as the `counteraprty_spendable_height`.