Enhance Window Evictor

Enhance the current functionality of Evictor as per this design document.

This includes:

• Allow eviction of elements from the window in any order (not only from the beginning). To do this Evictor must go through the list of elements and remove the elements that have to be evicted instead of the current approach of : returning the count of elements to be removed from beginning.
• Allow eviction to be done before/after applying the window function.

FLIP page for this enhancement : FLIP-4