moved SystemHealth monitor into Thread

since psutil.cpu_percent() will block...
fixed calculation of rest-time
process.poll() can fail for running process

so it's a bad check to determine whether the process is still up and running.
for now we just skip the test and call join/terminate regardless...
switching between no/communication with launched process

rather than hardcoding "True"...
run SystemHealth thead in daemon-mode

so it get's automatically shutdown wehn the process exits
added 'startCallback' to streaming.Server
use startCallback() for getting notified of the actual streaming URI
file-communication with streaming-server

the streaming-server will write the URI into a tempfile,
which we read and send back to the host via the startCallback().
this seems to work reliable.