implement LN anchor outputs

[SomberNight]

We should implement the option_anchor_outputs feature for lightning. (lightning/bolts#688)

[chrissv]

I am a recent "victim" of a commitment transaction having a too-low fee. I force-closed a channel to a now-defunct node, and the last channel activity (apparently a while ago) resulted in an updated commitment transaction being signed with 9.1 sat/vByte. So when that transaction was broadcast as part of the FC, it is languishing in the mempool. Since I don't own either the input or output addresses, I can't RBF or CPFP. If there were a dust-level output to one of my wallet addresses included in the transaction, I could at least CPFP that transaction to get it processed.

I'm not sure what to do now since I don't have any hope of the transaction being confirmed. Very disappointing (and expensive).