web: Replace waitress with wsgiref (stupid waitress...)

This was neccessary due to the way how waitress handles chunked
responses since v0.8.8: http://waitress.readthedocs.org/en/latest/#id3
This behaviour causes all larger files to be buffered in the temporary
directory, issuing a lot of writes that increase the storage wear. Since
this is unacceptable for our current RaspberryPi target, there was no
other possibility than to no longer use waitress. Until a better
alternative is found, the old wsgiref implementation will be used.