Bookkeeping of available resources of allocated slots in SlotPool.

In this version, a task will always requests slot with its own resource need. If the resource need is less than the default slot resource, it will always be allocated to a default sized slot. 

The extra resources in the slot leaves chances for other tasks within the same slot sharing group to fit in. To take these chance, SlotPool will maintain available resources of each allocated slot. Available resource of an allocated slot should always be the total resource of the slot minus resources of tasks already assigned onto the slot. In this way, the SlotPool would be able to determine whether another task can fit into the slot. If a task cannot fit into the slot, for slot sharing group the SlotPool should request another slot from the ResourceManager, and for colocation group it should fail the job.