Generic means for persisting system metadata.

Middleware or core features may need to store metadata
against accounts or containers. This patch adds a
generic mechanism for system metadata to be persisted
in backend databases, without polluting the user
metadata namespace, by using the reserved header
namespace x-<server_type>-sysmeta-*.

Modifications are firstly that backend servers persist
system metadata headers alongside user metadata and
other system state.

For accounts and containers, system metadata in PUT
and POST requests is treated in a similar way to user
metadata. System metadata is not yet supported for
object requests.

Secondly, changes in the proxy controllers ensure that
headers in the system metadata namespace will pass through
in requests to backend servers.

Thirdly, system metadata returned from backend servers
in GET or HEAD responses is added to the cached info
dict, which middleware can access.

Finally, a gatekeeper middleware module is provided
which filters all system metadata headers from requests
and responses by removing headers with names starting
x-account-sysmeta-, x-container-sysmeta-. The gatekeeper
also removes headers starting x-object-sysmeta- in
anticipation of future support for system metadata being
set for objects. This prevents clients from writing or
reading system metadata.

The required_filters list in swift/proxy/server.py is
modified to include the gatekeeper middleware so that
if the gatekeeper has not been configured in the
pipeline then it will be automatically inserted close
to the start of the pipeline.

blueprint cluster-federation

Change-Id: I80b8b14243cc59505f8c584920f8f527646b5f45
Merge "Generic means for persisting system metadata."