Fix postgres notification support in the QPSQLDriver.

Fix postgres notification support in the QPSQLDriver.

This patch fixes a critical bug in the qsqlpsql driver where
notifications aren't delivered when received. Any blocking libpq
function(specifically PQexec) will read all the incoming data
from the socket, including any pending notifications. This would
cause the socket notifier to never be fired for incoming
notifications that are already queued inside libpq. The qsqldriver
test case was skipping the postgres notification test because of
this bug, now its enabled and passing. In order to fix this
bug I made a wrapper function for PQexec in QPSQLDriverPrivate
that calls _q_handleNotification via QMetaObject::callMethod
QueuedConnection in order to deliver pending notifications
when control returns to the event loop. I also added a flag
to ensure only one call is made each time the event loop is
entered.

Change-Id: I19f5297094ae7ae46bfb0717e4fca744d69f7b92
Reviewed-by: Honglei Zhang <honglei.zhang@nokia.com>
Reviewed-by: Mark Brand <mabrand@mabrand.nl>