bake: Make pipeline processing multi-process. Not many changes here, as it's pretty straightforward, but an API change for processors so they know if they're being initialized/disposed from the main process or from one of the workers. This makes it possible to do global stuff that has side-effects (e.g. create a directory) vs. doing in-memory stuff.

from piecrust.serving.server import Server


def get_app(root_dir, sub_cache_dir='prod', enable_debug_info=False):
    server = Server(root_dir,
                    sub_cache_dir=sub_cache_dir,
                    enable_debug_info=enable_debug_info)
    app = server.getWsgiApp()
    return app