Summary:
When BufferCallback::onEgressBuffered() is called, applications usually want to
know how many bytes were buffered so that can take actions (such as avoiding
that connection, failing fast, etc).
This diff adds getTotalPendingBytes() to AsyncTransport to allow applications
to easily query how many bytes were actually buffered.

Reviewed By: spalamarchuk

Differential Revision: D13252677

fbshipit-source-id: 8ec203f6764e00f52d471321afb549376397eb84