Stem::Test::Echo - This cell accepts messages and sends back reply messages or command data. It can be used to test message receipt, replies, and command returns and to benchmark message throughput.

  [
          'class'	=>	'Stem::Test::Echo',
          'name'	=>	'test_echo',
  ],

This cell accepts various messages, all of which will echo some message back to the sender.

	An echo type message will do a reply with no data.

	An echo_data type message will do a reply with the sent data.

	An echo command message will return a null string.

	An echo_data command message will return the data.

	Any other message type or command will do a plain reply like
	an 'echo' type message.