Fix token purging for memcache for user token index.

When issuing a new token, purge all expired tokens from the user's
token index list.

New Options:
* max_compare_and_set_retry:
        The number of retries that will be attempted when performing
        an update of the user_record or the revocation-list record.
        This is relevant due to the use of CAS (compare and set)
        function of the memcache client. This allows for multiple
        keystone processes/wsgi/etc to run without worry of race
        conditions clobbering the lists.

DocImpact - New Options.

Change-Id: I9441105b1e46982b0354bccbf8297daaaa1904b2
Fixes: bug #1171985
Allow request headers access in app context.

- Give extensions access to request headers in app context.
- Implements: blueprint allow-access-to-headers-for-extension.

Change-Id: I22bba172a8ceaba9260487666172586c45aa95d4
Merge "Allow request headers access in app context."
Merge "Fix token purging for memcache for user token index."