Implement eager subscription in subcribeVia

Summary: Life-time of executor passed to subscribeVia can be tied to the previous stream. We have to eagerly subscribe to it to process onError/onComplete events and make sure we stop using that executor for any subsequent calls.

Reviewed By: phoad

Differential Revision: D7651264

fbshipit-source-id: c50090ff58a4835b439df2080e083a9302c40152

folly/io/async/ScopedEventBaseThread.h

@@ -36,7 +36,7 @@ typedef Range<const char*> StringPiece;
  * When the ScopedEventBaseThread object is destroyed, the thread will be
  * stopped.
  */
-class ScopedEventBaseThread : public IOExecutor {
+class ScopedEventBaseThread : public IOExecutor, public SequencedExecutor {
  public:
   ScopedEventBaseThread();
   explicit ScopedEventBaseThread(const StringPiece& name);