(WIP) Port ratelimit middlware from Swift

Adopt Swift's rate limiting middleware, altered to ratelimit on auth
tokens.  Some non relevant functionality such as per container limiting
has been removed.

Requires memcached to be present on the api node.  See
etc/glance-api-paste.ini for example pipeline and configuration.

Change-Id: I3df79ddbe7fbf8b6fc8bf9eeadb9fac59e202b90