[EchoClient] change the output format
Split output between two ini sections: global, clients. The global section includes results like: Arguments, total time spent in each phase, ... The clients section contains a sigle key 'csv' containing the csv data from each client inclufing the header: iterations, latency, reconnects, unexpected echos This leaves the post processing of the data to another program and does not hide experiment behavior (e.g. starvation of single clients) behind averages. Remove support for appending to an output file, because we do not use it in our emper-io-evaluation and it does not work anymore with the new output format.