Invert which proxy logging middleware instance logs

Prior to this patch the proxy_logging middleware always prepared to
read the response and process it for access logging, where the
proxy_logging instance first to handle the response bytes marked the
request as "logged".

This meant that the proxy_logging instance immediately before the
proxy-server app (right most in the pipeline, if properly setup) would
always log the responses for all the client requests, regardless if
middleware to the left of it still had processing to do for the
response. This would break SLO, where the slo middleware passed along
the GET of the manifest, unchanged from the client, so that the right
most proxy logging middleware would log the sponse for just the
manifest, but then the slo middleware would fetch all the segments of
the manifest, feeding them out in the response to the client, the
request now marked as "logged" so that the left most proxy logging
middleware would not log it.

This patch inverts this behavior so that now the first proxy_logging
middleware instance to receive a request in the pipeline marks that
request as handling it. So now, the left most proxy_logging middleware
handles logging for all client reaquests, and the right most
proxy_logging middleware handles all other requests initiated from
within the pipeline to its left.

Closes bug: 1297438

Change-Id: Ia3561523db76c693e4e0b2c38f461544dfcee086