Extract `Database` into its own `fuel-core-database` crate

The primary purpose of a separate crate for `Database` is to make the `fuel-core` cleaner. With the extraction of it, the `fuel-core` will have only the logic of connecting all services without mixing with the database. Also, maybe it will provide to us more flexibility regarding the dependencies with the ability to use ideas from https://github.com/FuelLabs/fuel-core/issues/529.

Another purpose is to make `Database` a reusable standalone crate with several different databases. It will have an abstraction that can be reused by `fuel-core` or other projects. It also will force us to think about `Database` as a reusable crate and introduce proper and clean API with comments.

Also, the crate could have performance benches and some utils for unit tests/benches.

It is related to the refactoring done by https://github.com/FuelLabs/fuel-core/issues/809, but because it is not related to ports and adapters, we have a separate issue for it.