Accounts service backed by kv-storage

[matdehaast]

Implement a peers service that is backed by kv-storage.

Service will need to handle:

• Serialisation account into value form for datastore
• Deserialisation into object from for service usage
• Implementation for rest of logic of peers service interface

May be worth having the balances of an account be under different keys so they can be updated without having to serialise/deserialise the whole accounts object on every balance update.