Network layer improved

From now, the client application is able to recieve multiple answers from the
server for every synchronous request it sends.

The only remaining weak spot is asynchronous requests (which may only contain
`CHAT` requests), which are still ignored for the moment.
Merge branch 'feature-network-layer' into feature-game-kernel

Synchronous requests are now working and will be very useful in the development
of the game kernel.