[twisted] add an option to configure twisted's threadpool size Twisted's threadpool size defaults to 10, which means we virtually try to serve up to 10 simulateneous requests but connection-pool size defaults to 4 on CW-server side. That means that under heavy load, chances are high that some HTTP queries end up with a 500 error code / connection-pool exhausted.

[book, form] include complete example of self-posted form with custom field/widget + minor other changes