Termination Correctness

[angelhof]

Address the issues shown in this script.

They refer to issues with the currect dataflow termination mechanism.

Copied here for completeness:

## This way of fixing the problem suffers from some issues.
##
## - First of all, gathering the children after the end of the graph
##   seems to gather more than just the alive nodes. This could lead
##   to killing some random pid in the system. This could potentially
##   be solved by gathering all pids incrementally.
##
## - In addition, this way of getting the last pid does not work if
##   there is more than one output. (This is never the case in our
##   tests, but could be.
##
## - Finally, it is not local, since all of the monitoring happens
##   globally. Ideally, it should be done by a wrapper in each -
##   node. The wrapper should monitor if the node dies, and if so it -
##   should send SIGPIPE to all its producers.

[angelhof]

Superseded by #77