Presearch Code Refactor

[CascadingRadium]

• Refactor the presearch code path to make it more generic and extensible.
• Add an ExtractFields API for obtaining the set of fields applicable to a generic query.
• Add support for a new API to set the index mapping for an alias.
  This is useful when an alias contains partitions of the same index, as the index
  mapping would be consistent across all indexes and can be inferred directly from
  the alias.

[Thejas-bhat]

my bad, rebased (to the latest master) but pushed on the wrong branch :p

[Thejas-bhat]

@CascadingRadium, so I looked into the bm25 code and realized that we are going to aggregate a field specific stat as well.

in your synonyms code, i see you've written a query tree parser

// ExtractSynonyms extracts synonyms from the query tree and returns a map of
// field-term pairs to their synonyms. The input query tree is traversed and
// for each term query, the synonyms are extracted from the synonym source
// associated with the field. The synonyms are then added to the provided map.
// The map is returned and may be nil if no synonyms were found.

can you refactor that to two sets of APIs one generic one which returns all the fields in the query and synonyms can use that API in ExtractSynonyms and bm25 can also use this API to fetch those field specific stat

and that API you can push it to this refactor branch

[abhinavdangeti]

Put a do not merge label here as this is already included within #2090 .

[abhinavdangeti]

-or- if you think we merge this first and rebase #2090 that's fine too.

[CascadingRadium]

i think we can merge this and rebase the #2090 on top

[abhinavdangeti]

Wait for one more review perhaps. Also @CascadingRadium hope you've made sure all the knn tests still succeed with this change.