probe /_all_dbs instead of the / welcome banner: couchdb serves the
banner publicly even on a secured 3.x, so a 200 there only proves an
instance is reachable. /_all_dbs is admin-gated by default since 3.0
(admin_only_all_dbs), so a 200 listing means the database names are
readable without auth on every version, while a secured server returns
401. the match requires a json array carrying a system database
(_users, _replicator or _global_changes), which keeps non-couchdb
arrays and prose clean.
no version is extracted: the /_all_dbs array carries no version string.
instances with renamed or deleted system databases are not matched.
