Shrink the scope of MemoryManager from TaskExecutor to slot

MemoryManager currently manages the memory bookkeeping for all slots/tasks inside one TaskExecutor. For better abstraction and isolation of slots, we can shrink its scope and make it per slot. The memory limits are fixed now per slot at the moment of slot creation. All operators, sharing the slot, will get their fixed fractional limits.

In future, we might make it possible for operators to over-allocate beyond their fraction limit if there is some available free memory in the slot but it should be possible to reclaim the over-allocated memory at any time if other operator decides to claim its fair share within its limit.