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.

htlcswitch/test_utils.go

@@ -1116,6 +1116,7 @@ func (h *hopNetwork) createChannelLink(server, peer *mockServer,
 			OnChannelFailure:        func(lnwire.ChannelID, lnwire.ShortChannelID, LinkFailureError) {},
 			OutgoingCltvRejectDelta: 3,
 			MaxOutgoingCltvExpiry:   DefaultMaxOutgoingCltvExpiry,
+			MaxFeeAllocation:        DefaultMaxLinkFeeAllocation,
 		},
 		channel,
 	)