make Inputter.getReceiverList() public
add Endpoint, EndpointImpl
change Node to use Endpoint, and take upstream Receiver in connect()
add SocketPair to testing code
JsonObjectInputter: fix initial freeze and nulls at end of input
 - create JsonParser on first call to readOneValue()
 - check result of readValueAsTree() for null or non-object JSON
add Handshaking utilities
add SsjpEndpoint, and Client and Server versions
 - plus a simple integration test