Call a configurable `wait_callback` in between dbsqlsend and dbsqlok -
this is a good place to do something like a gevent.sleep() because this
is after we've sent the query to the server and before we've received
the result
Add pymssql_gevent_test.py
Pass socket read file descriptor to wait_callback by using dbiordesc
function from FreeTDS. This allows the wait_callback to do
gevent.socket.wait_read and makes things much greener.
Add test for green query with gevent