[docdb] Support tablet splitting + xcluster replication

[hulien22]

Jira Link: [DB-300](https://yugabyte.atlassian.net/browse/DB-300)
Add support for handling tablet splitting in xcluster replication.
The plan is to support independent split decisions on the producer/consumer sides, which means that we may need to also make some improvements to our M:N tablet mappings for xcluster.

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List of work items:

• Add support for splitting on the producer ([xCluster + Tablet Splitting] Support splits on the producer #10874)
• Ensure that parent tablet is not deleted until WAL has been read (xCluster + Tablet Splitting: Ensure that split parent tablet can still be accessed by GetChanges #10296)
• Add support for calling GetChanges on a tablet parent in CheckTabletValidForStream (xCluster + Tablet Splitting: Ensure that split parent tablet can still be accessed by GetChanges #10296)
• Support a tablet split occurring on the consumer (xCluster + Tablet Splitting: Support tablet splitting on the consumer #10358)
• Properly remove parent tablet + metadata + metrics ([xCluster + Tablet Splitting] Improve parent tablet deletion #12476)
• Handle failures after split ([xCluster + Tablet Splitting] Handle failures during ProcessSplitTabletResult #11030)
• Add split children to cdc_state table and handle metrics for the new children tablets
• Add support for switching from a 1-1 mapping to a M:N mapping
• Re-enable splitting by default for cdc enabled tables (see [docdb] Splitting: Disable automatic splitting for 2DC enabled tables #8256)

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List of bug fixes:

• "Expected to find a write_request but were unable to" after delete operations with splitting ([xCluster][tablet-splitting] Replication is misbehaving after enabling source side tablet splitting.  #11299)
• [xCluster + Tablet Splitting] Children tablet checkpoints can be GC-ed by the time it is polled for #12164

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List of general improvements for M:N mappings:

• Improve the locality of pollers ([xCluster] Locality-Aware Placement for M:N tablet mapping in setup_replication #10186)
• Ability to see the producer->consumer tablet mapping (Already possible via get_universe_config)
• Ability to override CDCConsumerHandler threadpool size (xCluster: Allow for overriding the default CDCConsumerHandler threadpool size #10211)
• Add metrics to track write locality for pollers
• [YSQL] Correctly handle table/index splits in backup-restore #8229

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Follow ups

• Support tablet splitting for bootstrapping streams ([xCluster/Tablet Splitting] Support tablet splitting for bootstrapping streams #13170)

[hulien22]

Changes have been added to master and backported to 2.15.0 and 2.14