Added server side client ping function (called client_pong)

Set up a routine to send out "{}" frames every period. 

Potential TODOs: 
* Client tracks last message out and sends + interval afterwards (avoid needless pong frame)

Closes #174.
So, I was thinking about this one as well. Since I'm only checking the bool, there's the chance that the Worker would survive for however many minutes while the timer ticked down. The other option would be to create another shutdown channel that gets triggered and that cleans up this loop. (That would remove the mutex block as well.)