revocation events sync slows responses to all authenticated calls

[Adam Young]

There is a noticable lag when doing multiple calls to Keystone. The server shows in the log:

 KVS lock acquired for: os-revoke-events acquire /opt/stack/keystone/keystone/common/kvs/core.py:375

Putting the following delay in mitigates it significantly

        delta = datetime.timedelta(seconds=1)
        if self._last_fetch and self._last_fetch > timeutils.utcnow() + delta:
            return

[Dolph Mathews (dolph)]

Reminder- this is the last RC blocker without a patch in review ;)

[Adam Young (ayoung)]

Fixed by 7c9746c49b7518f2a000f7e4822e822c6eeee81d

[Dolph Mathews (dolph)]

Fixed committed in https://review.openstack.org/#/c/80441/ by moving to dogpile.cache in https://review.openstack.org/#/c/80441/5/keystone/contrib/revoke/core.py