Design the structure of the priority queue

[smb374]

• [x] Checkout the design from priority queue crate
• [ ] Design the storage for the internal structure
• [ ] Test the functionality

[smb374]

From garro95/priority-queue:

• The store of the internal data is hold by Store, which contains the following:
  1. map: A IndexMap from the indexmap crate
  2. heap: A Vec<usize> which is a binary heap for the heap sort
  3. qp: Position <-> Index
  4. size: size of heap
• We may directly make use of Pool from sharded-slab to achieve lock-free access for the interior structures.

[smb374]

~Another simple way is to wrap the current PriorityQueue with AtomicCell from crossbeam and enclose it with Pool. May try this out first before we start to implement.~ Doesn't work very well because AtomicCell requires T: Copy to use load & using fetch_update doesn't increase performance.

[smb374]

~Third option is use locks. Mutex from parking_lot have fair unlock which may achieve the same performance compare to the lock-free solution.~ Provide a worse performance in terms of throughput & transaction rate.

[smb374]

Currently implemented the HybridScheduler for a eclectic solution