We don't actually care if a confirmed transaction claimed other
outputs, only that it claimed a superset of the outputs in the
pending claim we're looking at. Thus, the variable to detect that
is renamed `is_claim_subset_of_tx` instead of `are_sets_equal`.

This function was very confusing - its used to determine by when
we have to stop aggregating this claim with others as it starts to
be at risk of pinning due to the counterparty's ability to spend
the output.

It is not ever used as a timelock for a transaction, and thus its
name is very confusing.

Instead we rename it `counterparty_spendable_height`.

This has never been used, and its set to a fixed value of zero for
HTLCs on local commitment transactions making it impossible to rely
on so might as well remove it.

Now that we don't store the confirmation height of the inputs
being spent, passing the current height to
`PackageTemplate::build_package` is useless - we only use it to set
the height at which we should next bump the fee, but we just want
it to be "next block", so we might as well use `0` and avoid the
extra argument. Further, in one case we were already passing `0`,
so passing the argument is just confusing as we can't rely on it
being set.

Note that this does remove an assertion that we never merge
packages that were crated at different heights, and in the future
we may wish to do that (as there's no specific reason not to), but
we do not currently change the behavior.

`PackageTemplate::get_height_timer` is used to decide when to next
bump our feerate on claims which need to make it on chain within
some window. It does so by comparing the current height with some
deadline and increasing the bump rate as the deadline approaches.

However, the deadline used is the `counterparty_spendable_height`,
which is the height at which the counterparty might be able to
spend the same output, irrespective of why. This doesn't make sense
for all output types, for example outbound HTLCs are spendable by
our counteraprty immediately (by revealing the preimage), but we
don't need to get our HTLC timeout claims confirmed immedaitely,
as we actually have `MIN_CLTV_EXPIRY` blocks before the inbound
edge of a forwarded HTLC becomes claimable by our (other)
counterparty.

Thus, here, we adapt `get_height_timer` to look at the type of
output being claimed, and adjust the rate at which we bump the fee
according to the real deadline.

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`.

For outbound HTLCs, the counterparty can spend the output
immediately. This fixes the `counterparty_spendable_height` in the
`PackageTemplate` claiming outbound HTLCs on local commitment
transactions, which was previously spuriously set to the HTLC
timeout (at which point *we* can claim the HTLC).

In a previous commit we updated the fee-bump-rate of claims against
HTLC timeouts on counterparty commitment transactions so that
instead of immediately attempting to bump every block we consider
the fact that we actually have at least `MIN_CLTV_EXPIRY_DELTA`
blocks to do so, and bumping at the appropriate rate given that.

Here we test that by adding an extra check to an existing test
that we do not bump in the very next block after the HTLC timeout
claim was initially broadcasted.