autopilot/agent: call ChannelBudget on constrainsts

We let the agent call ChannelBudget on its constraints directly, and
not go through the heuristic. This is needed since when we want to have
multiple active heuristics concurrently, it won't make sense anymore to
ask each of the heuristics.

The mockConstraints are also updated to act as the mockHeuristic did
before, by making it possible to control the responses it gives by
sending them on the contained channels.

autopilot/agent.go

@@ -478,12 +478,13 @@ func (a *Agent) controller() {
 		a.pendingMtx.Unlock()
 
 		// Now that we've updated our internal state, we'll consult our
-		// channel attachment heuristic to determine if we should open
-		// up any additional channels or modify existing channels.
-		availableFunds, numChans, needMore := a.cfg.Heuristic.NeedMoreChans(
+		// channel attachment heuristic to determine if we can open
+		// up any additional channels while staying within our
+		// constraints.
+		availableFunds, numChans := a.cfg.Constraints.ChannelBudget(
 			totalChans, a.totalBalance,
 		)
-		if !needMore {
+		if numChans == 0 {
 			continue
 		}