Clean up `PackageTemplate::get_height_timer` to consider type

Clean up `PackageTemplate::get_height_timer` to consider type

`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.