Replace `GrpcDriver` with `Driver` in the `start_driver()` function.

[panh99]

• Replaced GrpcDriver with Driver in start_driver()
• Replaced GrpcDriver with Driver in DriverClientProxy
• Removed sync lock
• Added ref_exit_flag = [False], which is used to "kill" the thread updating the client manager.
• Updated unit tests in driver/app_test.py accordingly (also accelerated tests by patching time.sleep).
• Changed name certificates to root_certificates