Added a priority queue for the request instead of push front

Now the request can be pushed to a high priority queue (instead of ocf_queue_push_req_front)
and to a low priority queue (instead of ocf_queue_push_req_back).
Both functions were merged into one function (ocf_queue_push_req) and instead of the
allow_sync parameter there is now a flags parameter that can be an OR combination of
OCF_QUEUE_ALLOW_SYNC and OCF_QUEUE_PRIO_HIGH

Signed-off-by: Ian Levine <ian.levine@huawei.com>
Signed-off-by: Robert Baldyga <robert.baldyga@huawei.com>

src/engine/engine_rd.c

@@ -42,7 +42,7 @@ static void _ocf_read_generic_hit_complete(struct ocf_request *req, int error)
 		OCF_DEBUG_RQ(req, "HIT completion");
 
 		if (req->error) {
-			ocf_queue_push_req_front_pt(req);
+			ocf_queue_push_req_pt(req);
 		} else {
 			ocf_req_unlock(c, req);