htlcswitch: cap fee updates to max fee allocation

In this commit, we begin to enforce a maximum channel commitment fee for
channel initiators when attempting to update their commitment fee. Now,
if the new commitment fee happens to exceed their maximum, then a fee
update of the maximum fee allocation will be proposed instead if needed.

A default of up to 50% of the channel initiator's balance is enforced
for the maximum channel commitment fee. It can be modified through the
`--max-channel-fee-allocation` CLI flag.

peer.go

@@ -585,6 +585,7 @@ func (p *peer) addLink(chanPoint *wire.OutPoint,
 		OutgoingCltvRejectDelta: p.outgoingCltvRejectDelta,
 		TowerClient:             p.server.towerClient,
 		MaxOutgoingCltvExpiry:   cfg.MaxOutgoingCltvExpiry,
+		MaxFeeAllocation:        cfg.MaxChannelFeeAllocation,
 	}
 
 	link := htlcswitch.NewChannelLink(linkCfg, lnChan)