Sort field tiebreaker for PIT (point in time) readers (#66093) #66588
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This commit introduces a new sort field called `_shard_doc` that can be used in conjunction with a PIT to consistently tiebreak identical sort values. The sort value is a numeric long that is composed of the ordinal of the shard (assigned by the coordinating node) and the internal Lucene document ID. These two values are consistent within a PIT so this sort criteria can be used as the tiebreaker of any search requests. Since this sort criteria is stable we'd like to add it automatically to any sorted search requests that use a PIT but we also need to expose it explicitly in order to be able to: * Reverse the order of the tiebreaking, useful to search "before" `search_after`. * Force the primary sort to use it in order to benefit from the `search_after` optimization when sorting by index order (to be released in Lucene 8.8. I plan to add the documentation and the automatic configuration for PIT in a follow up since this change is already big. Relates elastic#56828
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Backport of #66093