Add an optional advanced pool of memcached clients

This patchset adds an advanced eventlet safe pool of memcache clients. This
allows the deployer to configure auth_token middleware to utilize the new
pool by simply setting 'memcache_use_advanced_pool' to true. Optional
tunables for the memcache pool have also been added.

Co-Authored-By: Morgan Fainberg <morgan.fainberg@gmail.com>
Closes-bug: #1332058
Closes-bug: #1360446
Change-Id: I08082b46ce692cf4df449d48dac94718f1e98a6c
Merge "Add an optional advanced pool of memcached clients"