Telemetry Frontend gRPC NBI Re-structuring

- Changed the column type of the start and end timestamps to Float.
- Added ConvertRowToCollector() in TelemetryModel.
- Renamed the class variable from "DBobj" to "tele_db_obj".
- Renamed the local variable from "collector_id" to "collector_uuid".
- Added PublishStopRequestOnKafka() to publish the stop collector request on Kafka.
- Improved the test files.