htlcswitch+lnwallet+peer: default max htlc in fwding policy of new chans

In this commit, we set a default max HTLC in the forwarding
policies of newly open channels.

The ForwardingPolicy's MaxHTLC field (added in this commit)
will later be used to decide whether an HTLC satisfies our policy before
forwarding it.

To ensure the ForwardingPolicy's MaxHTLC default matches the max HTLC
advertised in the ChannelUpdate sent out for this channel,  we also add
a MaxPendingAmount() function to the lnwallet.Channel.

htlcswitch/link.go

@@ -71,6 +71,9 @@ type ForwardingPolicy struct {
 	// lifetime of the channel.
 	MinHTLC lnwire.MilliSatoshi
 
+	// MaxHTLC is the largest HTLC that is to be forwarded.
+	MaxHTLC lnwire.MilliSatoshi
+
 	// BaseFee is the base fee, expressed in milli-satoshi that must be
 	// paid for each incoming HTLC. This field, combined with FeeRate is
 	// used to compute the required fee for a given HTLC.