Refactor/database (#3150)

* refactor: split off KVStore from PersistentKeyDB

* chore: move low-level kv database into a separate module

(temporarily made some things public just to make it compile)

* chore: (roughly) split db tests into low and high level tests

* refactor: move migrations out of low level db

* refactor: better separation of concerns and clean-up

* refactor: move genesis hash and schema version methods

* chore: remove unneeded pub

* chore: fix clippy issues

* chore: rename prefix functions

* chore: rename persistent db tests module

* test: use all schemes in DB test

* chore: cargo fmt

---------

Co-authored-by: kylezs <zsembery.kyle@gmail.com>