WSGI env behaviour policies

When using uWSGI two different strategies can be used for allocating/deallocating the WSGI env using either the wsgi-env-behaviour or wsgi-env-behavior option:

cheat: it preallocates the env dictionary on uWSGI startup and clears it after each request. This is the default behaviour in uWSGI<=2.0.x

holy: creates and destroys the environ dictionary at each request. This is the default behaviour in uWSGI>=2.1