Possibly incorrect channel reserve referenced when updating feerate_per_kw

[valentinewallace]

Referencing this line: https://github.com/rust-bitcoin/rust-lightning/blob/9098240e341cf09578819cdf1d2950303478b137/lightning/src/ln/channel.rs#L1850

I believe instead of self.their_channel_reserve, it should be self.get_our_channel_reserve() (because we're checking if the remote is going below their channel reserve (required by us) by updating the fee).

Lmk if I've misunderstood. Related to #181.

[TheBlueMatt]

Yep! That's definitely wrong. In general our update_fee codepaths are super under-tested and under-reviewed, mostly because I'd rather remove them then bother testing them (lightning update_fee is kinda a bogus concept that exists as a stop-gap to anchor outputs, so I'm dubious of spending too much effort trying to get it exactly right, especially when its doubly hard to get right compared to the rest of the state machine).

[TheBlueMatt]

Closed by #613.