[services] Tunnelbroker - Avoid listing all entities in DatabaseEntities.

Summary:
Avoid listing all entities in DatabaseEntitiesTools.h and use a new approach like we already use in a Blob service.

Related linear task: ENG-706

Test Plan: Successfully built and run service using yarn run-tunnelbroker-service.

Reviewers: karol-bisztyga, palys-swm, jimpo

Reviewed By: karol-bisztyga, palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3524