refactor: Remove nChainTx from consensus

[maflcko]

nChainTx is returned by RPCs and is used to estimate (sync and rescan) progress.

However, it is also used for consensus and P2P logic to denote that a block and all its parents have BLOCK_VALID_TRANSACTIONS set.

This is confusing, because:

• It would be clearer to use the BlockStatus enum for this as well.
• It requires AssumeUtxo to fake the nChainTx values.
• It makes it harder to remove nChainTx completely, or limit it to code that needs it (estimate sync and scan progress and report it on RPC).

Fix it by introducing a new BLOCK_VALID_TRANSACTIONS_TREE level and use it where possible.

[DrahtBot]

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #29370 (assumeutxo: Get rid of faked nTx and nChainTx values by ryanofsky)
• #29355 (doc: Assert that assumed-valid blocks are not fully valid in CheckBlockIndex() by maflcko)
• #28120 (p2p: make block download logic aware of limited peers threshold by furszy)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

[sdaftuar]

I'm trying to understand if it's safe to modify these values -- I believe we write the nStatus field to disk and load it on restart, so wouldn't changing these values require a block-index-upgrade mechanism to be created, in order to avoid problems on restart?

[maflcko]

Good catch. It should be possible to keep the persisted format by using a serialize helper that translates nStatus both ways losslessly and then recover BLOCK_VALID_TRANSACTIONS_TREE in LoadBlockIndex as a memory-only value.

Though, I wonder if this pull request is still worth it, if more code changes are needed.

[maflcko]

Fixed. (Implemented my suggestion)

[maflcko]

Closing for now, but happy to reopen if someone thinks this is useful.