More Like This Class Structure Refactor [LUCENE-7802]

Currently the MoreLikeThis is a giant monolithic class, difficult to maintain.
Scope of this issue is to refactor the code, separating responsibilities of the main components and funcitonality :

- **MoreLikeThis facade** - the entry point
- **MoreLikeThisParams** - the huge set of params and defaults provided for the More Like This
- **Interesting Terms Retriever** - given a document in input this component has the responsibility of identifying interesting terms
- **Term Scorer** - given a term with document stats and collection stats, this module has the responsibility to score the term
- **Query Builder** - given a set of params and interesting terms, this component has the responsibility to build the Lucene query

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Migrated from [LUCENE-7802](https://issues.apache.org/jira/browse/LUCENE-7802) by Alessandro Benedetti (@alessandrobenedetti), 1 vote, updated May 01 2018
Linked issues:
- #9373
- #8549

Start by reading the existing MoreLikeThis implementation and the linked issues #9373 and #8549. Map the current responsibilities against the proposed MoreLikeThis facade, MoreLikeThisParams, Interesting Terms Retriever, Term Scorer, and Query Builder; the refactor is done when these responsibilities are separated while preserving the existing behavior.

Written by the indexing model from the issue text.