MEDIUM: filters: eval the backend instances first on the response path

MEDIUM: filters: eval the backend instances first on the response path

When the backend filters are attached to a stream, they are appended to the
global and request filter lists after the frontend ones, which is the
expected order: the request reaches the frontend filters first. But they
were also appended to the response filter list, while a response travels in
the opposite direction and reaches the backend filters first.

In the new filter mode, the backend instances are now prepended to the
response filter list, preserving their own response order, so a response is
evaluated by the backend instances before the frontend ones. The request
path is unchanged.

The regression tests relying on the response evaluation order of the
instance mode were updated accordingly.