MB-61029: Deferring the closing of vector index

[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 memory when the average of the hits is above a certain threshold 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 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.

[abhinavdangeti]

@Thejas-bhat I'd appreciate an illustration on how this cache management is being done with ewma tracker - perhaps on the ticket or a document which we can attach to the ticket.

[abhinavdangeti]

Looking almost ready @Thejas-bhat 👍🏼