add shutdownWrite to wangle for closing socket write only

Summary: current wangle has no way to close socket write side. writeFlags is extended so that wangle/AsyncSocketHandler can distinguish closeNow(), closeWithReset() and shutdownWrite()

Reviewed By: @viswanathgs

Differential Revision: D2522056

fb-gh-sync-id: 60616176d78311a4a09f9761a6ccda01dd502a88

folly/io/async/AsyncTransport.h

@@ -56,6 +56,10 @@ enum class WriteFlags : uint32_t {
    * will be acknowledged.
    */
   EOR = 0x02,
+  /*
+   * this indicates that only the write side of socket should be shutdown
+   */
+  WRITE_SHUTDOWN = 0x04,
 };
 
 /*