Extend client communication interface to also send training metadata

[ahellander]

To support more flexible passing of settings to clients, and to support gathering of more metadata / tracking from client executions, we want to extend the interface with the possibility to send (optional)metadata about a training / validation job to and from the client-combiner.

[ahellander]

Progress on this in branch "allow-stragglers".

[ahellander]

Now available.