Add method getCurrentlyAvailableNodes to QueryableGraph and Walkable Graph

It allows all graph implementations to return the list of nodes which are
immediately available to be fetched. NOTE: Not-currently-available here does
not mean the nodes do not exist in the graph. It simply means they are not
ready to be fetched immediately yet.

--
MOS_MIGRATED_REVID=137701432
diff --git a/src/test/java/com/google/devtools/build/skyframe/NotifyingHelper.java b/src/test/java/com/google/devtools/build/skyframe/NotifyingHelper.java
index 6cde573..559b3cf 100644
--- a/src/test/java/com/google/devtools/build/skyframe/NotifyingHelper.java
+++ b/src/test/java/com/google/devtools/build/skyframe/NotifyingHelper.java
@@ -100,6 +100,11 @@
         throws InterruptedException {
       return notifyingHelper.wrapEntry(key, delegate.get(requestor, reason, key));
     }
+
+    @Override
+    public Iterable<SkyKey> getCurrentlyAvailableNodes(Iterable<SkyKey> keys, Reason reason) {
+      return delegate.getCurrentlyAvailableNodes(keys, reason);
+    }
   }
 
   static class NotifyingProcessableGraph