Fix iteration over competitive iterators [LUCENE-9565]

#10320 introduced a sort optimization where documents can be skipped.
But iteration over competitive iterators was not properly organized,
as they were not storing the current docID, and
when competitive iterator was updated the current doc ID was lost.

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Migrated from [LUCENE-9565](https://issues.apache.org/jira/browse/LUCENE-9565) by Mayya Sharipova (@mayya-sharipova), updated Oct 07 2020
Pull requests: https://github.com/apache/lucene-solr/pull/1952

Start by reviewing the sort optimization introduced in #10320 and the competitive-iterator implementation to trace how the current docID is stored and updated. Confirm that updating a competitive iterator preserves the current docID, and check the existing pull request #1952 for prior implementation context and expected behavior.

Written by the indexing model from the issue text.