lnwallet+htlcswitch: Introduce a fee buffer.

We take into account a fee buffer of twice the current fee rate
of the commitment transaction plus an additional htlc output
when we are the opener of the channel hence pay when publishing the
commitment transaction. This buffer is not consensus critical
because we only consider it when we are in control of adding a
new htlc to the state. The goal is to prevent situations
where we push our local balance below our channel reserve due to
parallel adding of htlcs to the state. Its not a panacea for these
situations but until we have __option_simplified_update__ deployed
widely on the network its a good precaution to protect against
fee spikes and parallel adding of htlcs to the update log.

Moreover the way the available balance for a channel changed.
We now need to account for a fee buffer when we are the channel
opener. Therefore all the tests had to be adopted.

htlcswitch/link.go

@@ -1423,6 +1423,11 @@ func (l *channelLink) handleDownstreamUpdateAdd(pkt *htlcPacket) error {
 	// commitment chains.
 	htlc.ChanID = l.ChanID()
 	openCircuitRef := pkt.inKey()
+
+	// We enforce the fee buffer for the commitment transaction because
+	// we are in control of adding this htlc. Nothing has locked-in yet so
+	// we can securely enforce the fee buffer which is only relevant if we
+	// are the initiator of the channel.
 	index, err := l.channel.AddHTLC(htlc, &openCircuitRef)
 	if err != nil {
 		// The HTLC was unable to be added to the state machine,