Make async function for listen and send in ConnectionHandler

[chaeynz]

The goal for the listen() function is to be non-blocking.
Every message that is received should be printed straight to the command line for now.
It should also allow multiple connections at the same time, to prevent issues for this case.

The send() function should be asynchronous aswell.

The main problem is the listen() function.

[chaeynz]

In 3138e94 I added a test for those functions.

The test for the sync versions of these functions pass.

I am not entirely sure if the test for the async versions is implemented correctly, but worth a try :D
This should make it easier to verify the intended behaviour