Closing std indput pipes of `DispatchableJob`s

Siavash discovered that subprocess standard input pipes were not
being closed after the subprocess launched by the `start` method
of `DispatchableJob` instances. This was leading to an accumulation
of open files over iterations, eventually leading to an Exception
if enough iterations were run.

As per Siavash's recommendation, I am now closing these pipes
with `self.process.stdin.close()` in the `wait` method of
`DispatchableJob` where the subprocess returns.