multi: update internals to support updating max htlc

In this commit, we update the router and link to support users
updating the max HTLC policy for their channels. By updating these internal
systems before updating the RPC server and lncli, we protect users from
being shown an option that doesn't actually work.

routing/router.go

@@ -219,6 +219,10 @@ type ChannelPolicy struct {
 	// TimeLockDelta is the required HTLC timelock delta to be used
 	// when forwarding payments.
 	TimeLockDelta uint32
+
+	// MaxHTLC is the maximum HTLC size including fees we are allowed to
+	// forward over this channel.
+	MaxHTLC lnwire.MilliSatoshi
 }
 
 // Config defines the configuration for the ChannelRouter. ALL elements within