Resolve config for a namespaced key

Returns the config stored for a single namespaced key, merged over its
namespace base and the connection's global base — most specific value
wins per top-level key.

Pass exactly one namespace query parameter. The namespace is whatever you
choose (brand, store, region, …); the value is your own identifier
for it. ?brand=xxx-yyyy reads the key brand:xxx-yyyy, merged over
brand:_base and _base.