Move all DLO functionality to middleware

This is for the same reason that SLO got pulled into middleware, which
includes stuff like automatic retry of GETs on broken connection and
the multi-ring storage policy stuff.

The proxy will automatically insert the dlo middleware at an
appropriate place in the pipeline the same way it does with the
gatekeeper middleware. Clusters will still support DLOs after upgrade
even with an old config file that doesn't mention dlo at all.

Includes support for reading config values from the proxy server's
config section so that upgraded clusters continue to work as before.

Bonus fix: resolve 'after' vs. 'after_fn' in proxy's required filters
list. Having two was confusing, so I kept the more-general one.

DocImpact

blueprint multi-ring-large-objects

Change-Id: Ib3b3830c246816dd549fc74be98b4bc651e7bace
Merge "Move all DLO functionality to middleware"