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In summary, this flip has 2 goals:

• Annotate SingleThreadFetcherManager as PublicEvolving.
• Shield FutureCompletingBlockingQueue from users and limit all operations on FutureCompletingBlockingQueue in FetcherManager.

Public Interfaces

Change SingleThreadFetcherManager, SplitFetcherManager,  SplitFetcher and SplitFetcherTask from Internal  to PublicEvolving .

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The Connectors that utilize constructors (including param elementsQueue) of SingleThreadMultiplexSourceReaderBase, SourceReaderBase, and SingleThreadFetcherManager should be migrated to the new constructor that does not have elementsQueue parameter.

Mark the impacted constructors as deprecated in 1.19, and remove them in release of 2.0.

Test Plan

nothing else to do.

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