tikvpv, kvrpcpb: Add RawCompareAndDelete API

[healthwaite]

This commit re-implements compare-and-delete by creating a new RawCompareAndDelete API. Previously compare-and-delete piggy-backed on the RawCompareAndSwap API by introducing a delete flag into the RawCASRequest message. As described in issue tikv/tikv#19462 this can cause silent data corruption if a newer client is talking to a server which doesn't support compare-and-delete. In this PR:

• Add RawCADRequest/Response messages
• Add RawCompareAndDelete RPC
• Deprecate delete flag in RawCASRequest: in practice we could actually just remove this field as no version of the server of client has been release which makes use of it. Let me know if you'd prefer to remove it.

Accompanying tikv PR: tikv/tikv#19465
Accompanying issue: tikv/tikv#19462

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