Remove connections coupling with providers in database

[jpraynaud]

Why

We need to make the providers capable of returning an iterator alongside of a collection. The providers are currently using the database connection which prevents this operation to be done easily (in particular because of lifetime issues).

What

Remove the connection from the all the providers in the Mithril nodes and rename them to a more appropriate name (as they will not provide anything anymore).

How

• Remove connection from the providers in mithril-persistence and rename to query
• Remove connection from the providers in aggregator and rename to query
• Remove connection from the providers in signer and rename to query

Later

• Assess the existing repositories where we could benefit from an iterator (memory consumption optimization)
• Implement the iterators to optimize memory consumption