This change introduces a single parameter, MemcacheUseAdvancedPool,
to enable usage of advanced connection pool in keystone middleware.
This is useful to avoid bursting connection to memcached.
Note that the default value of memcached_use_advanced_pool was changed
from false to true during Xena cycle[1], so this parameter is no longer
required in master. However the change in keystonemiddleware will
never be backported. This change is created so that we can switch to
advanced pool even in older releases.
[1] https://review.opendev.org/c/openstack/keystonemiddleware/+/773939
Closes-Bug: #1931047
Change-Id: I2887249af44ccfdae1592dd9120d3366fa059876
(cherry picked from commit 09bcacd25a)
9a89b9add6