[EchoClient] check for termination more repeatly and remeber the client termination time

* Merge the two echo functions echo and linkedEcho into a single templated
  Client member function _run(). This reduces code duplication but the resulting
  echo loop is less readable because of more if constexpr switches.
* Move more code out of the echo loop into separate functions:
  * unexpected echo message
  * on ECONNRESET handling
  * error handling
  * termination condition