[wallet] [zk] Properly introduce CWallet::IncrementNoteWitnesses / DecrementNoteWitnesses

[ch4ot1c]

These two methods need to be added as a first step into the Mempool logic for JoinSplits.

https://github.com/zcash/zcash/blob/13b41a6760c67524dd77afc67e7d556ec1ea71be/src/wallet/wallet.cpp#L452

CWallet::ChainTip has been removed in the latest Bitcoin code, and significant nearby refactoring has taken place. We should consider using their latest improvements.

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[ch4ot1c]

Additionally, ClearNoteWitnessCache needs to be properly placed (again - nearby refactoring):

https://github.com/zcash/zcash/blob/cd0484e023cdb4f1ba8e6ed284a0a0232ca9a4cf/src/init.cpp#L1657

[ch4ot1c]

The ChainTip signal is replaced with ChainStateFlushed in Bitcoin.