[mongo] close old pymongo connections
[mongo] close old pymongo connections

Summary:
try/except close old pymongo connection before creating new one
auditing code to see if client can buildup connections

Blame:

Test Plan: monitor connections

Reviewers: bpalsson, vitaliy, rli, andrew

Reviewed By: andrew

Differential Revision: https://phab.wish.com/D42348

ContextLogic/mongoengine,ContextLogic/mongoengine