Move file-backed entries back into the database

[LesnyRumcajs]

Issue summary

Currently, we store a few entries, like GENESIS, HEAD and meta.yaml outside of the database. After the DB re-organization, we have a dedicated column for such data.

share/forest/mainnet
❯ ls
GENESIS  HEAD  meta.yaml  paritydb/

It'd be great to move them into the DB and potentially get rid of the File-backed wrapper. To reduce the SettingsStore trait proliferation, it may make sense to pass to, e.g., ChainStore "another" DB in the form of a trait object. There may be other solutions.

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