Add more methods for the in-memory db

[ferranbt]

📝 Summary

This PR extends the current MockDB with an InmemoryDB implementation. InMemoryDB is an implementation of the DatabaseService interface. It does not implement all the methods of the interface but enough of them to have a functional mev-boost-relay service. This is the database that we have been using in https://github.com/flashbots/builder-playground to run mev-boost-relay in memory without Postgresql.

I also fixed the lint in a few places which were not part of this PR.

⛱ Motivation and Context

For quick testing scenarios, it is easier to run mev-boost-relay without Postgresql and with a temporal database.

📚 References

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✅ I have run these commands

• make lint
• make test-race
• go mod tidy
• I have seen and agree to CONTRIBUTING.md