The compiler will anyway insert padding to ensure the size of the
structure is a multiple of 8. That will under Valgrind show up as
writes of uninitialized data when the messages are sent over the
pipe.
Rather than forcing the structure to be aligned on 4 byte boundary
we simply expand the id field to an 8 byte type.
MessageQueue encapsulates a message queue built on top of a
pipe. The message queue needs a handler for receiving messages
and must be added to a worker for pumping messages through the
pipe.
Each Worker will have an instance of MessageQueue.
