resources: Overhaul legacy code around the resource API

This patch makes a number of changes in the engine surrounding the
resource API. In particular:

* Cleanup of send/read event.
* Cleanup of DoSend (now Event) in the Watch method.
* Events are now more consistently pointers.
* Exiting within Watch is now done in a single place.
* Multiple incoming events will be combined into a single action.
* Events in flight during an action are played back after CheckApply.
* Addition of Close method to API

This gets things ready for rate limiting and semaphore metaparams!

pgraph/pgraph.go

@@ -567,7 +567,7 @@ func (g *Graph) GraphSync(oldGraph *Graph) (*Graph, error) {
 	for v := range oldGraph.Adjacency {
 		if !VertexContains(v, vertexKeep) {
 			// wait for exit before starting new graph!
-			v.SendEvent(event.EventExit, true, false)
+			v.SendEvent(event.EventExit, nil) // sync
 			oldGraph.DeleteVertex(v)
 		}
 	}