Consider Order class as a domain model type

The order class was placed in `protocol.messages.brokerage` package, since it
was considered as part of a protocol message. But this type is actually
representing an entity of the Coinffeine domain model. Thus, the class has been
moved to the `common` package until we have a better place to store our
domain model classes.
Introduce an interface to the broker in Coinffeine API

The `CoinffeineApp` trait, which provides the API of a Coinffeine peer,
lacks a way to interact with the broker. This change provides such an interface
and its default implementation, which uses the `PeerActor` instance to
request the submission of orders.