onchaind: Adjust the sweep target deadline for fee estimation

We used to always target `now() + 300`, which ends up never really
confirming, as the fee estimate bumps into the min-relay-fee
limit. With this commit we set an absolute target of 2 weeks, and a
linear fee rampup, until we are at T-2h, at which point we just stick
with the estimate, and try with this increased feerate to try and get
the sweep confirmed.

This ought to make RBF transactions much more efficient for closing
channels.

Changelog-Changed onchaind: When sweeping funds from channel closes we now use an absolute deadline of 2 weeks, rather than a relative deadline which causes sweeps not confirm for prolonged times.