The following test code never checked anything because the if statement was
always false:

    if (cs != &chainman_restarted.ActiveChainstate()) {
        BOOST_CHECK_EQUAL(cs->m_chain.Height(), 109);
    }

Also, the height of the background chainstate it was intending to check is 110,
not 109. Fix both problems by rewriting the check.

Make Chainstate objects aware of what block they are targeting. This makes
Chainstate objects more self contained, so it's possible to determine what
blocks should be downloaded and connected to them without needing to query
ChainstateManager or look at other Chainstate objects.

The motivation for this change is to make code more readable, so it only
requires knowing about Chainstate targets, not reasoning about the assumeutxo
snapshot validation sequence. This change also enables simplifications to the
ChainstateManager interface in subsequent commits, and could make it easier to
implement new features like creating new Chainstate objects to generate UTXO
snapshots or index UTXO data.

Get rid of m_disabled/IsUsable members. Instead of marking chains disabled for
different reasons, record chainstate validity and validation progress
explicitly and use that information directly.

Move duplicate code from ChainstateManager::ActivateSnapshot and
ChainstateManager::ActivateExistingSnapshot methods to a new
ChainstateManager::AddChainstate method.

The "AddChainstate" method name doesn't mention snapshots even though it is
only used to add snapshot chainstates now, because it becomes more generalized
in a later commit in this PR ("refactor: Add ChainstateManager::m_chainstates
member")

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.

Use to simplify code determining the chainstate leveldb paths.

CurrentChainstate() is basically the same as ActiveChainstate() except it
requires cs_main to be locked when it is called, instead of locking cs_main
internally.

The name "current" should also be less confusing than "active" because multiple
chainstates can be active, and CurrentChainstate() returns the chainstate
targeting the current network tip, regardless of what chainstates are being
downloaded or how they are used.

ValidatedChainstate() accessor replaces GetChainstateForIndexing() with no
change in behavior.

Change ChainstateRole parameter passed to wallets and indexes. Wallets and
indexes need to know whether chainstate is historical and whether it is fully
validated. They should not be aware of the assumeutxo snapshot validation
process.

IsSnapshotActive() method is only called one place outside of tests and
asserts, and is confusing because it returns true even after the snapshot is
fully validated.

The documentation which said this "implies that a background validation
chainstate is also in use" is also incorrect, because after the snapshot is
validated, the background chainstate gets disabled and IsUsable() would return
false.

IsSnapshotValidated() is only called one place outside of tests, and is use
redundantly in some tests, asserting that a snapshot is not validated when a
snapshot chainstate does not even exist. Simplify by dropping the method and
checking Chainstate validity field directly.

SnapshotBlockhash() is only called two places outside of tests, and is used
redundantly in some tests, checking the same field as other checks. Simplify by
dropping the method and using the m_from_snapshot_blockhash field directly.

Use to replace m_active_chainstate, m_ibd_chainstate, and m_snapshot_chainstate
members.

Deduplicate code looping over chainstate objects and calling
ActivateBestChain().

Just use m_chainstates array instead.