CASSANDRA-21156 Avoid initializing system keyspaces during legacy threshold conversion

[cheeeee]

CASSANDRA-21156: setting the legacy table_count_warn_threshold (or keyspace_count_warn_threshold) makes
startup fail with ExceptionInInitializerError. The YAML converter for these keys calls
SchemaConstants.getLocalAndReplicatedSystemTableNames(), which initializes SystemKeyspace and the other
system keyspace classes; their static initializers read DatabaseDescriptor, which is still being loaded,
and throw NullPointerException.

Changes:

• The per-keyspace table-name sets are declared in SchemaConstants and built only from the String
  constants of SystemKeyspace, TraceKeyspace, AuthKeyspace, SystemDistributedKeyspace and AccordKeyspace.
  Referencing a compile-time constant does not trigger class initialization (JLS 12.4.1), so the
  converter no longer initializes any keyspace class. The TABLE_NAMES fields of those classes now alias
  the SchemaConstants sets, so there is a single definition.
• Converters: the two thresholds share one clamp. Values <= 0 pass through unchanged in both directions
  (guardrail disabled). A legacy value that is not larger than the number of system tables/keyspaces now
  converts to 0 with a warning, i.e. warn on any user creation; it used to convert to a zero or negative
  number. This is a semantics change for such (unusually small) legacy values.

Testing:

• TableCountThresholdToGuardrailConverterTest (fresh JVM, no configuration loaded): converts both
  thresholds and asserts DatabaseDescriptor.getRawConfig() is still null afterwards, plus the boundary
  values of the clamp. Without the change it fails with the ExceptionInInitializerError from the ticket
  (SystemKeyspace. via SchemaConstants via Converters).
• SystemTableNamesConsistencyTest: each SchemaConstants set equals the table names of the corresponding
  keyspace metadata, so the hoisted constants cannot drift from the real schema.
• AccordJournalDaemonYamlInitializationTest, AccordJournalToolYamlInitializationTest (each in a fresh
  JVM): the journal_sai index on system_accord.journal is still configured after YAML initialization in
  daemon and in tool mode.

CASSANDRA-21156