[Segment Replication] Merge to main tracking - Refactor InternalEngine to support Segrep specific Engine implementation.

[mch2]

The current InternalEngine implementation wires up an IndexWriter to index into lucene and set merge policies. With segment replication we should not be wiring up a writer at all for replicas.

This issue involves refactoring the existing InternalEngine to extract needed functionality to write to the transaction log so that it can be reused inside of a segrep specific engine implementation.

The segrep engine should be able to write to the xlog when index() or delete() operations are invoked. It should also use a new OpenSearchReaderManager implementation that is similar to lucene's SegmentInfosSearcherManager that refreshes when new SegmentInfos are provided.