make AsyncSocket::WriteRequest an interface

Summary: This will allow a subsequent diff to implement file transfers as another type of write request

Test Plan: unit

Reviewed By: davejwatson@fb.com

Subscribers: net-systems@, folly-diffs@, yfeldblum, chalfant, fugalh, bmatheny

FB internal diff: D2080257

Signature: t1:2080257:1432044566:bcc0724d349879f46e3e58ee672aff7bf37fa5f6

folly/io/async/AsyncSocket.h

@@ -517,6 +517,7 @@ class AsyncSocket : virtual public AsyncTransportWrapper {
   };
 
   class WriteRequest;
+  class BytesWriteRequest;
 
   class WriteTimeout : public AsyncTimeout {
    public: