MB-66295: avoid redundant calculation of bm25 metrics

[Thejas-bhat]

• cache a map which maps the field to its cardinality value so that we don't create the fieldDict everytime
• Furthermore in the case all the threads have a cache miss, use the write lock to make sure that we create the fieldDict only once to minimize garbage.
• Use a separate mutex construct so that the contention is localised to the metrics gathering and doesn't affect the other threads' TFR generation.
• MB-66295: Introduce BM25Reader interface  bleve_index_api#68

[abhinavdangeti]

Looks good @Thejas-bhat, nice one.

[abhinavdangeti]

Already reviewed by @CascadingRadium , so will move ahead.