refactor: simplify state with queued system

[louis-jan]

Description

• Deprecate shared single_queue_busy state & SerialTaskQueue since it would not work with cont_batching = true and n_parallel > 1.
• Deprecate instance's is_stopped state, better using inferenceStatus enum.
• Introduce ConcurrentTaskQueue to handle tasks with the workers number = n_parallel.
• No complicated state handling, put into the queue and it just work (inference & embedding).

Test results

• n_parallel = 1 & cont_batching = false

  |        T1          |       T2           |       T3          |       T4          |
  |--------------------|--------------------|-------------------|-------------------|
  | Send chat comp.    | Send chat          | A1 completed      | A2 completed      |
  | request A1         | comp. request A2   | A2 started        |                   |
  |                    |                    |                   |                   |
  |                    |                    |                   |                   |
  

• n_parallel = 2 & cont_batching = true

  |      T1      |      T2      |       T3       |       T4       |                T5               |
  |--------------|--------------|----------------|----------------|--------------------------------|
  | Send chat    | Send chat    | A1 & A2 are    | Send chat      | A1 & A2 are                    |         
  | completion   | completion   | running        | completion     | completed                      |                            
  | request A1   | request A2   |                | request A3     | A3 started running             |                 
  

[tikikun]

LGTM