Migrate stop search to new DB

Summary:
Use the NextGenDB instead of the old one to search stops
Remove singleton instance of NextGenDB that screws up the database lock

Test Plan: Build and run in whatever environment is possible

Reviewers: valerio.bozzolan

Reviewed By: valerio.bozzolan

Subscribers: valerio.bozzolan, Libre BusTO

Tags: Libre BusTO

Differential Revision: https://gitpull.it/D40