Refactor BrokerActor

Make BrokerActor
 
 * Use an initialization message
 * Extract timeout managing logic to ExpirationSchedule class
 * Move magic number to ProtocolConstants
 * Make traded currencies a configurable value
Use GatewayProbe on BrokerActorTest
Merge pull request #115 from sortega/feature/simplify_cake_2

Refactor BrokerActor to simplify cake pattern