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.

peer.go

@@ -1670,15 +1670,17 @@ out:
 				continue
 			}
 
-			// We'll query the localChanCfg of the new channel to
-			// determine the minimum HTLC value that can be
-			// forwarded. For fees we'll use the default values, as
-			// they currently are always set to the default values
-			// at initial channel creation.
+			// We'll query the localChanCfg of the new channel to determine the
+			// minimum HTLC value that can be forwarded. For the maximum HTLC
+			// value that can be forwarded and fees we'll use the default
+			// values, as they currently are always set to the default values
+			// at initial channel creation. Note that the maximum HTLC value
+			// defaults to the cap on the total value of outstanding HTLCs.
 			fwdMinHtlc := lnChan.FwdMinHtlc()
 			defaultPolicy := p.server.cc.routingPolicy
 			forwardingPolicy := &htlcswitch.ForwardingPolicy{
 				MinHTLC:       fwdMinHtlc,
+				MaxHTLC:       lnChan.MaxPendingAmount(),
 				BaseFee:       defaultPolicy.BaseFee,
 				FeeRate:       defaultPolicy.FeeRate,
 				TimeLockDelta: defaultPolicy.TimeLockDelta,