This helper was intended to create a set of missing IDs, but it didn't.

Release note: None

We need a version gate before switching over to the span configs
infrastructure by default (cockroachdb#73876, future commit). In a mixed-version
cluster we need to wait for the host tenant to have fully populated
system.span_configurations (read: reconciled) at least once before it's
safe to using it as a view for all split/config decisions.

    clusterversion.EnsureSpanConfigReconciliation

We also want to ensure that the view over all configs maintained
per-store is at least as up-to-date as some full reconciliation
timestamp.

    clusterversion.EnsureSpanConfigSubscription

Without a version gate, it would be possible for a replica on a
new-binary-server to apply static fallback configs (assuming no entries
in `system.span_configurations`), in violation of explicit configs
directly set by the user. Though unlikely, it's also possible for us to
merge all ranges into a single one (with no entries in
system.span_configurations, the infrastructure can erroneously conclude
that there are zero split points).

To get past all this, we author a three-step migration, two with
attached migrations for each of the steps above (in order), and a final
cluster version gate.

    clusterversion.EnableSpanConfigStore

Release note: None

We replace the 'enable-span-configs` cluster option with its opposite
without actually using it anywhere. In a future commit we'll flip
the (opt-in) EnableSpanConfigs flag with an opt-out variant, using this
new cluster option to power it. Update the relevant logictest files
while here.

Release note: None

We were previously using opt-in flags to enable span configs; this
commit flips it into opt-out ones (DisableSpanConfigs and
COCKROACH_DISABLE_SPAN_CONFIGS). This has the following effects:
a. fresh crdb nodes/clusters run with span configs enabled.
b. our entire test suite runs using the span configs infrastructure, save
   for a select few written:
   (i)  with the gossipped system config span in mind;
   (ii) using legacy testing harnesses that prevent injection of
        spanconfig dependencies due to circular imports.

We also introduce a more minified form of the opt-out testing flag:
UseSystemConfigSpanForQueues. Using the minified form still enables the
SQL-side of span configs to run (Reconciler, SQLWatcher, SQLTranslator)
but defaults to using the system config span down in KV.

Release note: None

Few (old) packages use localtestcluster package for tests, a package
that previously opted out of using the span configs infrastructure due
to circular dependency reasons. With \cockroachdb#74865 this is no longer
necessary.

  $ dev test pkg/kv/kvclient/kvcoord pkg/kv/kvserver/idalloc
  [...]
  //pkg/kv/kvclient/kvcoord:kvcoord_test            PASSED in 54.7s
  //pkg/kv/kvserver/idalloc:idalloc_test            PASSED in 2.0s

  INFO: Build completed successfully, 10 total actions

Release note: None

A few tests rely on RANGE DEFAULT's configuration to also apply to the
scratch range keyspace, an out-of-the box behavior for the system config
span. This PR introduces an opt-in testing knob to do the same.

Release note: None

It's unclear why these tests experience a massive slowdown.
Investigating it is tracked as part of cockroachdb#74919.

Release note: None