This PR is the second part of the pure state sync refactoring,
encapsulating `StateSyncMetadata` as a separate entity. Now it's pretty
straightforward what changes are needed for the persistent state sync as
observed in the struct `StateSync`:

- `state`: redirect directly to the DB layer instead of being
accumulated in the memory.
- `metadata`: handle the state sync metadata on disk whenever the state
is forwarded to the DB, resume an ongoing state sync on a restart, etc.

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Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Alexandru Vasile <60601340+lexnv@users.noreply.github.com>