There's no need to have this member live on CChainState since it's only used
in validation.cpp.
https://github.com/bitcoin/bitcoin/pull/16194/commits/55d525ab9004631d30dcc60a1ec5d9cd6c6afe56
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Depends on D6967
Partial backport of Core [[https://github.com/bitcoin/bitcoin/pull/16194 | PR16194]]