James O'Beirne c29f26b47b validation: add CChainState::m_disabled and ChainMan::isUsable
and remove m_snapshot_validated. This state can now be inferred by the
number of isUsable chainstates.

m_disabled is used to signal that a chainstate should no longer be used
by validation logic; it is used as a sentinel when background validation
completes or if the snapshot chainstate is found to be invalid.

isUsable is a convenience method that incorporates m_disabled.
2023-02-22 12:13:11 -05:00
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