Extract database feature from common crate

[Alenar]

Why

We want to reduce the dependencies needed by our projects, reduce the number of features of mithril-common and make our codebase more manageable by splitting code in cohesive blocks.

What

Extract all store & database related code from mithril-common to a new local crate . This crate doesn't needs to be published to crates.io since the mithril-client doesn't use those features.

How

• Create a new local crate (name to be defined, ie: mithril-database or mithril-store ?)
• Add it as dependency to the nodes that needs it (should be only mithril-aggregator and mithril-signer)
• Move the database & store related code from mithril-common to this new crate
• Update the imports in impacted project to target the new crate
• Remove the database feature from mithril-common