Ensure that:
* Storage config might be removed.
* Local FS GC process is set up when Local FS storage is configured.
* Local FS GC process gets its timer reset on config updates.
* Storage / exporter gets chosen based on `type` only.
* Exporter config updates propagated as before.
Also employ `emqx_ft_schema:translate/1` instead of duplicating
defaults where applicable.
The exporter is responsible for keeping fully transferred and
successfully assembled files. This was on the local storage itself
before. This abstraction is needed to give us an ability to
support S3 destinations more easily, just by swapping the storage
exporter.
Also implement local filesystem exporter and reimplement parts of
the `emqx_ft` API on top of it.
Ilya Averyanov