gfapi: use native STACK_WIND for read _async() calls

There is little value in using synctask wrappers for async IO
requests, as STACK_WIND is asynchronous by nature already.

Skip going through synctask for read/write async calls.

Change-Id: Ifde331d7c97e0f33426da6ef4377c5ba70dddc06
BUG: 1009134
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5897