Ampoule serializes job execution

[moshez]

Each process in an Ampoule process pool will only be given one job at a time to work on.

All of the pieces are in place to support concurrent async job execution within each child, except that the ProcessPool class intentionally serializes job distribution. There should at least be an option to allow more than one job to be sent to a worker process at once.

[glyph]

I definitely like having serialized job execution be the default (spawn more processes for more parallel stuff!) but yeah this should be tunable.