Make backend container requests use the same X-Timestamp

The proxy.controllers.base's generate_request_headers will set an X-Timestamp
header for you if it didn't get populated by additional kwarg or the
transfer_headers method.  This works fine if you only call it once per
request, but because of how proxy.controllers.obj and
proxy.controllers.container fill in the backend update header chains in
_backend_requests we need multiple independent copies and call the base
controllers generate_request_headers once of each backend request - which left
the ContainerController sending down different X-Timestamp values
(microseconds apart) for PUT and DELETE.

The ObjectController skirts the issue entirely because it always preloads a
X-Timestamp on the req used to generate backend headers, and it allows it to
be copied over via transfer_headers by including 'x-timestamp' in it's
pass_through_headers attribute.

Because the container-replicator is already does merge_timestamps the
differences would always eventaully even out and there is no consistency bug,
but this seems cleaner since they put_timestamp being stored on the three
replicas during a container PUT were all coming from the same client request.

Since both PUT and DELETE were effected, and the ContainerController doesn't
need to allow X-Timestamp to pass_through like the ObjectController does for
container-sync, it seemed cleanest to fix the issue in _backend_requests via
the additional kwarg to generate_request_headers.

There's a driveby fix for FakeLogger and update to the proxy_server's
ContainerController tests.

Change-Id: Idbdf1204da33f8fb356ae35961dbdc931b228b77
Merge "Make backend container requests use the same X-Timestamp"