Returns the address that this message is associated with.
If this object represents a message to be sent, this address
is the recipient's address.
If this object represents a message has been received, this address
is the address of the sender.
If the address for the message isn't set, null
is returned.
Note: Sending responses to a received message
is allowed easily by this design through reusing the same
Message
object with the replaced payload.
Unless a special processing of the address is required by the mesaging
protocol that is used, the address field can be left untouched.