Simplify TFO write path

Simplify TFO write path

Summary:
We currently call handleInitialReadWrite.
The reason for this is that if the read callback
was set before TFO was done with connecting, then
we need to call handleinitialreadwrite to setup the
read callback similar to how connect invokes handleInitialReadWrite
after it's done.

However handleinitalreadwrite may also call handleWrite
if writeReqHead_ is non null.
Practically this will not happen since TFO will happen on
the first write only where writeReqHead_ will be null.

The current code path though is a little bit complicated.
This simplfies the code so that we dont need to potentially
call handleWrite within a write call.

We schedule the initial readwrite call asynchrously.
The reason for this is that handleReadWrite can actually fail if updating
events fails. This might cause weird state issues once it returns and we
have no mechanism of processing it.

Reviewed By: djwatson

Differential Revision: D3695925

fbshipit-source-id: 72e19a9e1802caa14e872e05a5cd9bf4e34c5e7d