TopDocsCollector Should Not Depend on Priority Queue [LUCENE-8877]

TopDocsCollector is tightly coupled to the notion of priority queue, which is not necessarily a good abstraction to have since the collector really just needs an interface to iterate on and hold docID and score, with possibly shard indexes.

We should rewrite this to a more simplistic interface with priority queue being the default implementation

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Migrated from [LUCENE-8877](https://issues.apache.org/jira/browse/LUCENE-8877) by Atri Sharma (@atris), updated Jun 26 2019

Start by tracing TopDocsCollector and its dependency on the priority queue. Determine the interface needed to iterate over docID, score, and possible shard indexes, then verify that priority queue remains the default implementation; the issue names no files or tests.

Written by the indexing model from the issue text.