list os-service causing reconnects to memcached all the time

Description
===========

we are running a victoria openstack cluster (python3). and I observe that everytime when an openstack compute service list executed, nova-api will create a new connection to memcache. Actually there are several reasons to cause this behavior

1. when running natively with eventlet's wsgi server, everytime a new coroutine is created to host web request and this causes keystonemiddle auth_token which uses python-memcached to reconnect to memcahced all the time
2. os-services will trigger nova.availability_zones.set_availability_zones and it will update cache every time, since cellv2 is enabled, this method is running in an co-routine as well
3. python-memcached's Client is inheriting from threading.local which will be monkey_patched to use eventlet's implementation and thus for every co-routine context it will create a new connection

Steps to reproduce
==================

1. Patch def _get_socket and print connection
2. execute openstack compute service list

Expected result
===============

Maintain stable connections to memcached

Actual result
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Reconnects

Environment
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1. devstack victoria openstack