[services] Tunnelbroker - Introducing deleteQueueIfEmpty method for DeliveryBroker

Summary:
Following the comment on D3833 introducing the deleteQueueIfEmpty method. The method purpose is if clientDeviceID messages queue is empty we don't
need to store folly::MPMCQueue for it and need to free memory to fix possible 'ghost' queues.

Linear task: ENG-1146

Test Plan: Run yarn run-tunnelbroker-service, service is built and run after.

Reviewers: karol-bisztyga, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4100