Prior to this change, the global (a big map) config is saved
to persistent_term as one single term.
With this change, configs are keyed by their name spaces,
i.e. the root struct names in hocon schema.
This is to allow loading configs with more dynamic scope,
so we can start emqx app independently (without all merged configs),
and to allow external plugins to load their configs incrementally
at run-time.
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