Database improvements

[D4nte]

Few things on the DB:

• Quickcheck tests
• Store values in CBOR instead of JSON: The main reason is that it's a binary format because the DB stores byte arrays. It made less sense to use JSON than CBOR but there is little different considering the usecase.
• flush the db when storing a swap (arguably the most import patch of the PR)

[D4nte]

bors r+

[bors[bot]]

Build succeeded:

• build_test (macos-latest)
• build_test (ubuntu-latest)
• static_analysis