Moved the Carrier* interfaces to beam-common in order to implement them (partially) also in beam-client.
Moved the ClientCarrier implementation to beam-common since MQTT is used in both projects, beam-server and beam-client. However, the implementation of ServerCarrier remains in beam-server since that is only needed there.
Adapted to changes in beam-common where the Server constructor changed (adde MQTT address). Also, simplified the config keys.