Puts dbuser in params, for correct use by Windshaft renderer cache.
Before this fix, and after commit 1c9f63c9, the renderer cache key
did not contain the db user.
If the request is authenticated (with map_key) then we log as the
database owner, otherwise we log as the default user.
The default user is now "publicuser" by default.
Raises dependency on Windshaft to 0.4.9+, to get the grainstore
version allowing override of database username.
Add test for req2params function, particularly authentication,
Add test for authenticated / unauthenticated access