Implement relational store in aggregator

[jpraynaud]

Issue

Implement the new relational design of the aggregator store to prepare strong foundations on which we will build the feature of signing generic data. In this phase, we will not need to modify the OpenAPI, which means that this modification must be transparent for the signer and client nodes

Here is the schema that needs to be implemented:

Note: At first, we will process open messages sequentially, but the system will be evolved to a parallel runtimes setup (one for each type of message) later

Methodology

• Prepare migration script (data structure creation + data migration) + (destruction previous store, maybe later)
• Update the existing store (adapter modification only) if it is possible
• Create a new store otherwise
• Adapt/refactor runtime and/or http server to use the new store

To do

• Migrate/adapt stake_pool tables #787
• Create a Stake Distribution service #799
• Migrate/adapt epoch_setting table #813
• Migrate/adapt certificate table #817
• Create signed_entity_type table #815
• Create open_message table #827
• Migrate/adapt signer_registration table #828
• Migrate/adapt signed_entity table #816
• Create signer table #814
• Migrate/adapt single_signature table #829