[wsgi] make sure request.content is available for consumption
The wsgi.input stream is not seekable, so make a copy either to memory
or disk so it's still available to the user even after we've parsed it
(for POST form processing). Closes #3554996
usr/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/