reinstate disconnection from mongo in unicorn master thread

https://github.com/edx/cs_comments_service/commit/e61430d9438abf1db64fcac0eb9dd3e0f18a07bd was inadvertently lost in a recent change, this puts it back.
(cherry picked from commit 30945ddf9d09f6b0b715b97e172ee547c4feb821)