Random computation echoserver

To expose the benefits of io-stealing or a completer thread it seams helpful to have a echo server with differnt computation times. So that outstanding work on workers currently executing long computations can be reintroduced to the system via io-stealing or the completer improving the overall throughput and reducing latencies.

This implementation is really simple and biased.

Possible improvements:

• Use thread_local random number generator to produce even distribution.
• Use only two fixed times and select one of both with a given probability.