Store Block Range Merkle roots in signer and aggregator databases

[jpraynaud]

Why

In order to provide faster signature and proof generation of the Cardano transactions, we need to store and use Block range Merkle trees roots in the databases of the signer and aggregator.

What

As soon as transactions of a block range are committed to the store, the corresponding block range root needs to be computed and stored in a new table of the store.

How

• [ ] Split TransactionImporter trait in separate traits with clear responsibilities:
  • one the has responsibility for storing transactions and block range roots (which will probably not called from the signable builder in a near future in order to warmup and avoid blocking signature process)
  • one (or more) with responsibility of computing/storing block range roots
• [ ] Implement concrete implementations of new traits