Ampoule serializes job execution

[Jean-Paul Calderone]

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.

[Jean-Paul Calderone (exarkun)]

Here's an example program which shows the serialization that happens. There should be a way for this program to complete in about 30 seconds, rather than 450 seconds.