Upserts a config field on the given connection (string → config, blob →
configdata). The upsert keys on lower(field) (see the SQL), so it is
idempotent and case-insensitive.
Reads a SYSTEM (server-wide) config field from the global admin database.
For server-wide keys (infra URLs, locale) and the bootstrap reads that run
before any user is resolved. Per-user config goes through User::config.