set the "tasks_by_state" method as optional

Driver API was changed with the new requirement in 75c4708:
the new method "tasks_by_state" must be defined in a queue
driver now. We want to be user-friendly, so we allow user
to work without implementing this method in the customer driver.

Part of #126

Author: LeonidVas

queue/abstract.lua

@@ -75,6 +75,12 @@ local function tube_release_all_tasks(tube)
         return
     end
 
+    -- Custom drivers may not have an implementation of
+    -- the "tasks_by_state" method.
+    if tube.raw['tasks_by_state'] == nil then
+        return
+    end
+
     log.info(prefix .. 'releasing all taken task (may take a while)')
     local released = 0
     for _, task in tube.raw:tasks_by_state(state.TAKEN) do