Follow-up to #33259, which added GetBackgroundVerificationProgress() and exposed it in getblockchaininfo.
getchainstates still reports verificationprogress relative to the network tip for every chainstate. This modifies verificationprogress to report the correct value, relative to
getchainstates reports verificationprogress relative to the network tip for
every chainstate. A chainstate validating the chain underneath a snapshot
stops at the snapshot base, so its value saturates below 1.0 and the
chainstate disappears from the list before reaching it.
Expose GetBackgroundVerificationProgress, which is relative to that target
block, as a separate optional field rather than changing the meaning of the
existing one.
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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36219.
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