Remove the last PriorityQueue subclasses

[thecoop]

After the various PRs changing PriorityQueue to use Comparator or LessThan functional interfaces, there are only a few subclasses remaining before we can make PriorityQueue final:

• TermsMergeQueue. This has some additional state in the stack variable
• FieldValueHitQueue. This is further subclassed, and is public (although experimental)
• HitQueue. Public class (although internal). Used in several difference places, so this class represents some common functionality.
• TopDocs.ScoreMergeSortQueue, TopDocs.MergeSortQueue. Has some significant state fields in them
• TopOrdAndNumberQueue and subclasses. Part of the public API, and has some abstract behavious set by subclasses
• SuggestWordQueue. Public API

How should we handle these?

[msokolov]

I'm not entirely sure what the purpose of making PriorityQueue final is, but you could fork it into a PriorityQueueFinal, make that one final and use it in the places where we don't extend PriorityQueue?

[thecoop]

I've changed PriorityQueue to use LessThan by default in #14873, which resolves several of these. I'm not sure if it's worth the API changes to make PriorityQueue actually final?