The stream_manager is now the sole owner of the child process handle and
is responsible for cleaning it.
The stream_manager expects a shutdown command from the broadcast channel
it receives as parameter and watches it concurrently with the child process
I/O.
When the shutdown command is received, the child process is killed and the
remaining I/O is processed. Then the child process is reaped. Or it is
killed and reaped after EOF is encountered, would the child process exit
before (or at the same time as) the shutdown command is issued.
