[Backport MB-56879 to 7.2.1] Reduce overheads on low configuration nodes

Indexer data pipeline overheads can be reduced for low configuration nodes. E.g.

Consider a Capella configuration of 3 nodes with 16 Gig ram each, 48G total. With data + index co-located, the quota available is 80% i.e. 12.8GB. Index service gets 50% = 6.3GB. If data+index+query is co-located, index quota = 4.2GB.

With such configuration, it is better to reduce allocations throughout the pipeline which are generally done to achieve throughput of the order of 500k ops/sec.

Configs to consider:

indexer.mutation_manager.moi.fracMutationQueueMem
indexer.settings.minVbQueueLength
indexer.settings.sliceBufSize

This can either be done dynamically or Capella control plane can push the configuration based on pre-defined rules as all the settings are already configurable.

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In 7.2.1 time frame we are trying to check if we can optimise the values of the parameters above to save or better utilise the memory dynamically changing them is not in the scope.