Further improve memory of FieldInfos [LUCENE-6630]

In #7386, FieldInfos were improved to use an array except in very sparse cases. In the sparse cases, a TreeMap is still used. However, even in the sparse case, we can still use an array, and then use binary search. The perf will be equivalent, but memory use should be much less.

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Migrated from [LUCENE-6630](https://issues.apache.org/jira/browse/LUCENE-6630) by Ryan Ernst (@rjernst), 1 vote
Attachments: [LUCENE-6630.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-6630/LUCENE-6630.patch)

Start by locating the FieldInfos implementation and reviewing the changes from #7386, along with the attached LUCENE-6630.patch. Compare the sparse TreeMap path with the existing array path and identify how the binary-search behavior should be validated. Done means the sparse case uses the proposed lower-memory representation without a performance regression.

Written by the indexing model from the issue text.