pgraph: Add fast pausing and exiting of graphs

This causes a graph to actually stop processing part way through, even
if there are poke's that want to continue on. This is so that the user
experience of pressing ^C actually causes a shutdown without finishing
the graph execution. It might be preferred to have this be a user
defined setting at some point in the future, such as if the user presses
^C twice. As well, we might want to implement an interrupt API so that
individual resource execution can be asked to bail out early if
requested. This could happen on a third ^C press.

pgraph/pgraph.go

@@ -58,6 +58,7 @@ type Graph struct {
 	Adjacency map[*Vertex]map[*Vertex]*Edge // *Vertex -> *Vertex (edge)
 	Flags     Flags
 	state     graphState
+	fastPause bool        // used to disable pokes for a fast pause
 	mutex     *sync.Mutex // used when modifying graph State variable
 	wg        *sync.WaitGroup
 	semas     map[string]*semaphore.Semaphore
@@ -129,6 +130,7 @@ func (g *Graph) Copy() *Graph {
 		wg:        g.wg,
 		semas:     g.semas,
 		slock:     g.slock,
+		fastPause: g.fastPause,
 
 		prometheus: g.prometheus,
 	}