Relax constraints for inbound feerate updates on anchor channels

Relax constraints for inbound feerate updates on anchor channels

Channels supporting anchors outputs no longer require their feerate
updates to target a prompt confirmation since commitment transactions
can be bumped when broadcasting. Commitment transactions must now at
least meet the minimum mempool feerate, until package relay is deployed,
such that they can propagate across node mempools in the network by
themselves.

The existing higher bound no longer applies to channels supporting
anchor outputs since their HTLC transactions don't have any fees
committed, which directly impact the available balance users can send.