Commands able to use this parallelisation are `stop`, `kill` and `rm`.
We're using a backported function from python 3, to allow us to make
the most of a pool of threads without having to write the low level
code for managing this ourselves.
A default value for number of threads is a low enough number so it
shouldn't cause performance problems but if someone knows the
capability of their system and wants to increase it, they can via
an environment variable DEFAULT_MAX_WORKERS
Signed-off-by: Mazz Mosley <mazz@houseofmnowster.com>