Added HS_MQTTID field that contains the MQTT id/username for handshake. This is needed to identify different devices that use the same public key (e.g. one user (person) that has several computers running Beam with the same account).
Updated handshake content field names. Removed the prefix because the message type already defines the exact purpose of every field. Therefore, the content field 'signature' may also be used as signature in other situations, etc..