[wsgi] avoid reading the entire request body in memory
Import POST form handling code from
https://raw.github.com/defnull/multipart/master/multipart.py to avoid
reading arbitrary amounts of data from the network in memory.
NOTES:
- In the twisted case we limit the max request content-length to 100MB (by
default), which seems kind of arbitrary, but avoids this issue
- werkzeug.formparser has suitable code as well, but I don't know if we want to
add it as a dependency