Python from 3.2 on support timeouts on subprocesses directly
subprocess32 backports that to Python 2

Easier to read. Also, `timeout=None` because that's what modern
subprocess expects

There are many caveats, but at least Ctrl-C now works as expected.

There were out-of-order writes to the persitence file when there were
multiple threads. This confines file-writing to a dedicated thread that
works a Queue.

The existing parallel scheduler behaves as is
(With common behavior lifted to a new base clase).

A new parallel scheulder maintains a work queue and the workers retrieve
directly from there.

a distinction is made for py2 vs py3, to make use of the built-in thread
pool executor when possible.

No new magic so far