Always lock the DBus dispatcher before dbus_connection_send*

We lock it before dbus_connection_send_with_reply (the async version) in
QDBusConnectionPrivate::sendWithReplyAsync. We weren't locking it before
send_with_reply_and_block and we apparently should. The locking around
the dbus_connection_send function might not be necessary, but let's do
it to be safe.

The lock now needs to be recursive because we may be inside
QDBusConnectionPrivate::doDispatch.

Task-number: QTBUG-42189
Change-Id: I7b6b350909359817ea8b3f9c693bced042c9779a
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>