Fix an easy to fail unit-test

 - The test has race condition. The mutual discovery
   doesn’t works well as one runnable can be finished
   and not discoverable anymore before another one
   tries to discover.
 - Switch to use an Echo server / client runnable,
   which only the client needs to discover the server.

This closes #77 on GitHub.

Signed-off-by: Terence Yim <chtyim@apache.org>