Warn if read_affinity is configured but not enabled

To get the proxy's read affinity to work, you have to set both
"read_affinity = <stuff>" and "sorting_method = affinity" in the proxy
config. If you set the first but not the second, then you don't get
read affinity, and Swift doesn't help you determine why not.

Now the proxy will emit a warning message if read_affinity is set but
sorting_method is a value other than "affinity", so if you check your
logs to see why it isn't working, you'll get a hint.

Note that the message comes out twice per proxy process, so with 2
workers you'll see the warning 6 times on startup (2 for master + 2 *
2 per worker). It's sort of annoying, but at least it's not
per-request.

Bonus docstring fix: remove a sentence that's not true

Change-Id: Iad37d4979a1b7c45c0e3d1b83336dbcf7a68a0c9
Move all SLO functionality to middleware

This way, with zero additional effort, SLO will support enhancements
to object storage and retrieval, such as:
 * automatic resume of GETs on broken connection (today)
 * storage policies (in the near future)
 * erasure-coded object segments (in the far future)

This also lets SLOs work with other sorts of hypothetical third-party
middleware, for example object compression or encryption.

Getting COPY to work here is sort of a hack; the proxy's object
controller now checks for "swift.copy_response_hook" in the request's
environment and feeds the GET response (the source of the new object's
data) through it. This lets a COPY of a SLO manifest actually combine
the segments instead of merely copying the manifest document.

Updated ObjectController to expect a response's app_iter to be an
iterable, not just an iterator. (PEP 333 says "When called by the
server, the application object must return an iterable yielding zero
or more strings." ObjectController was just being too strict.) This
way, SLO can re-use the same response-generation logic for GET and
COPY requests.

Added a (sort of hokey) mechanism to allow middlewares to close
incompletely-consumed app iterators without triggering a warning. SLO
does this when it realizes it's performed a ranged GET on a manifest;
it closes the iterable, removes the range, and retries the
request. Without this change, the proxy logs would get 'Client
disconnected on read' in them.

DocImpact

blueprint multi-ring-large-objects

Change-Id: Ic11662eb5c7176fbf422a6fc87a569928d6f85a1
Merge "Warn if read_affinity is configured but not enabled"
Merge "Move all SLO functionality to middleware"