Optimization: don't wait on a Deferred already in State.DONE.

When a Callback returned a Deferred already in State.DONE, the current
Deferred was paused and immediately resumed, which involved the creation
of an unnecessary Continue callback and other objects.  We now bypass
this entire code path, which helps in the cases where a Callback returns
a Deferred built with Deferred.fromResult or Deferred.fromError, or if a
real Deferred completes so quickly that we directly see it in State.DONE.

Change-Id: I710266e05b5f37d676018f706c923ecafa276ffc

Author: tsuna

src/Deferred.java

@@ -1143,6 +1143,17 @@ private void handleContinuation(final Deferred d, final Callback cb) {
         + ", the result returned was the same Deferred object.  This is illegal"
         + ", a Deferred can't run itself recursively.  Something is wrong.");
     }
+    // Optimization: if `d' is already DONE, instead of calling
+    // adding a callback on `d' to continue when `d' completes,
+    // we atomically read the result off of `d' immediately.  To
+    // do this safely, we need to change `d's state momentarily.
+    if (d.casState(State.DONE, State.RUNNING)) {
+      result = d.result;  // No one will change `d.result' now.
+      d.state = State.DONE;
+      runCallbacks();
+      return;
+    }
+
     // Our `state' was either RUNNING or DONE.
     // If it was RUNNING, we have to suspend the callback chain here and
     // resume when the result of that other Deferred is available.