We were previously using an opt-in flag to enable span configs; this
commit flips it into an opt-out one. This has the following effects:

• fresh crdb nodes/clusters run with span configs enabled.
• our entire test suite runs using the span configs infrastructure, save
  for a select few written:
  • with the gossipped system config span in mind;
  • 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