SlotPool should check slot id when accepting a slot offer with existing allocation id

For flip-6, there may be two or more slot assigned to the same slot allocation. For example, taskExecutor1 register, and assign allocationID1 to its slot1, but from taskExecutor1 side, the registeration timeout, and it register again, RM will fail the allocationID1 and assign slot2 on taskExecutor2 to it. but taskExecutor1 has already accept the allocationID1. 

So taskExecutor1 and taskExecutor2 both offer slot to jobmaster with the allocationID1. Now slot pool just accept all the slot offer, and this may one slot leak.