[EchoClient] reconnect on ECONNRESETC and C++-ify the code

* Unconditionally reconnect when the TCP connection was reset and count the reconnects
* Send Client id and iteration in each echo
* Use a Client class holding a clients state instead of a pthread style clientFunc
* Create the Client object in the client Fiber to use NUMA first-touch policy
* Reduce code duplication:
  * starting a new Client is now done with Client::startNew
  * latency measurement and error handling are moved from the clientIteration*
    functions in the Client::run function