The main task of the thread, called in a loop as long as
Service.isRunning() returns true. Overridden methods are responsible for
yielding or pausing the thread when it's idle. The method must also not
block indefinitely so that a call to Service.stop() is able to
gracefully terminate the thread.
Processes a single incoming message by sending it to the appropriate
internal location. If an incoming message contains an MSA-2 field, it is
assumed that this message is meant as a reply to a message that has been
sent earlier. In this case an attempt is give the message to the object
that sent the corresponding outbound message. If the message contains an
MSA-2 but there are no objects that appear to be waiting for it, it is
discarded and an exception is logged. If the message does not contain an
MSA-2 field, it is concluded that the message has arrived unsolicited. In
this case it is sent to the Responder (in a new Thread).