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.

# This is a utility module that can be used with any WSGI-compatible server
# like Werkzeug or Gunicorn. It returns a WSGI app for serving a PieCrust
# website located in the current working directory.
import os
from piecrust.wsgiutil import get_app


root_dir = os.getcwd()
app = get_app(root_dir)
# Add this for `mod_wsgi`.
application = app