Remove hardcoded references to m_ibd_chainstate and m_snapshot_chainstate so
MaybeCompleteSnapshotValidation function can be simpler and focus on validating
the snapshot without dealing with internal ChainstateManager states.
This is a step towards being able to validate the snapshot outside of
ActivateBestChain loop so cs_main is not locked for minutes when the snapshot
block is connected.
Get rid of m_disabled/IsUsable members. Instead of marking chains disabled for
different reasons, store chainstate assumeutxo status explicitly and use that
information to determine how chains should be treated.
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.
Ryan Ofsky