Requires changes in HealthMessage.initialize() and getDocument().
HealthMessage.getDocument() should write the HealthMessage.Entry.sequenceId into
its XML document representation.
HealthMessage.initialize() should read the senders sequence id for health
message.entry from XML document representation.
Currently, the receiver side is just creating a sequence id based on
order of receiving messages. Jxta messaging protocol does not guarantee
that messages are received in precise order that they were sent, so the
current sequencing mechanism could be resulting in out of order processing
of health messages. This could result in incorrect computed cache state for an
instance in the master node.