minimize using of the "pk" index of the "_queue_taken" space

In the future, we will refuse to use the "pk" index of the
"_queue_taken" space to improve performance (the conclusion about
the improvement in performance is based on the benchmarks that
will be added in one of the next commits). So, let's minimize
its use.

Needed for #85

Author: LeonidVas

queue/abstract.lua

@@ -79,8 +79,8 @@ end
 -- connection id == "conn_id".
 -- Throw an error if task is not take in the session.
 local function check_task_is_taken(tube_id, task_id, conn_id)
-    local _taken = box.space._queue_taken:get{conn_id, tube_id, task_id}
-    if _taken == nil then
+    local _taken = box.space._queue_taken.index.task:get{tube_id, task_id}
+    if _taken == nil or _taken[1] ~= conn_id then
         error("Task was not taken in the session")
     end
 end
@@ -166,7 +166,7 @@ function tube.ack(self, id)
 
     self:peek(id)
     -- delete task
-    box.space._queue_taken:delete{conn_id, self.tube_id, id}
+    box.space._queue_taken.index.task:delete{self.tube_id, id}
     local result = self.raw:normalize_task(
         self.raw:delete(id):transform(2, 1, state.DONE)
     )
@@ -180,7 +180,7 @@ local function tube_release_internal(self, id, opts, connection_id)
     opts = opts or {}
     check_task_is_taken(self.tube_id, id, connection_id)
 
-    box.space._queue_taken:delete{connection_id, self.tube_id, id}
+    box.space._queue_taken.index.task:delete{self.tube_id, id}
     self:peek(id)
     return self.raw:normalize_task(self.raw:release(id, opts))
 end
@@ -202,7 +202,7 @@ function tube.bury(self, id)
     local conn_id = connection.id()
     local is_taken, _ = pcall(check_task_is_taken, self.tube_id, id, conn_id)
     if is_taken then
-        box.space._queue_taken:delete{conn_id, self.tube_id, id}
+        box.space._queue_taken.index.task:delete{self.tube_id, id}
     end
     if task[2] == state.BURIED then
         return task
@@ -428,7 +428,7 @@ function method._on_consumer_disconnect()
         tube = box.space._queue.index.tube_id:get{task[2]}
         if tube == nil then
             log.error("Inconsistent queue state: tube %d not found", task[2])
-            box.space._queue_taken:delete{task[1], task[2], task[3] }
+            box.space._queue_taken.index.task:delete{task[2], task[3]}
         else
             log.warn("Consumer %s disconnected, release task %s(%s)",
                 id, task[3], tube[1])