cache eviction policy for vector indexes

[Thejas-bhat]

• Uses the exponentially weighted moving average to get the average hits on a particular field (thereby a particular vector index). The index stays in the cache above a certain threshold of the average hits and below which the index is closed and the memory is freed for reuse on C side of things.
• This ensures that we are not keeping the index in the cache (and holding up memory) even when there is no query workload on the field in a segment so the memory pressure does get reduced on the C side of operations.
• Still a work in progress

[Thejas-bhat]

Closing this in favour of #226