Support groups inside xqueue

[ochaton]

Sometimes there is a necessity to process group of tasks in sequence.

Ex. consumer puts task1 and then task2 into group1 . xqueue must gurantee for consumers that task2 never be given for consumers until task1 is processed correctly.

Task is processed correctly only if it was acknowledged (removed from space, moved to Done state, or temporary left as zombie).

Request for comments:

0. Space for xqueue must have field order which will be used by xqueue for ordering tasks inside each group.
1. Introduce new status L (for Locked) which assigns to each task when producer puts it into existing group.
2. Implement transitions of status L -> R for method :ack.
3. If task put with delay into existing group, it must be delayed from the last task in the group.
4. If taken task released by consumer with delay back inside group, then all tasks of the group must be proper delayed.
5. If task is buried then neither other task from the group should be processed