lnwallet/channel: add fee updates to update logs
lnwallet/channel: add fee updates to update logs

Instead of special casing the UpdateFee messages, we instead add them to
the update logs like any other HTLC update message. This lets us avoid
having to keep an extra set of variables to keep track of the fee
updates, and instead reuse the commit/ack logic used for other updates.

This fixes a bug where we would reset the pendingFeeUpdate variable
after signing our next commitment, which would make us calculate the new
fee incorrectly if the remote sent a commitment concurrently.

When restoring state logs, we also make sure to re-add any fee updates.

LightningNetwork/lnd,aakselrod/lnd,aakselrod/lnd,LightningNetwork/lnd