This commit introduces a NetworkMessage class. This class has two
subclasses - InboundMessage and OutboundMessage. These messages can
be serialized and deserialized independent of the transport. This allows
more granular testing. Additionally, the serialization mechanism is now
a simple Supplier. This builds the framework to eventually move the
serialization of transport messages to the network thread. This is the
one serialization component that is not currently performed on the
network thread (transport deserialization and http serialization and
deserialization are all on the network thread).
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