JTI: Optimize telemetry subscription path

Junos telemetry interface (JTI) supports enhancements to increase the sensor subscription installation speed for collectors. Whether a dynamic sensor subscribe or unsubscribe request from a collector uses remote procedure calls (gRPC) services or gRPC Network Management Interface (gNMI) services to make the request, resource paths (sensors) in the request are individually validated and committed. The following enhancements shorten the subscription installation process and time: 

•  Validation is no longer done using the ephemeral database's configuration load operation. 
•  Network Agent instead uses information from sensor YANGs and the Packet Forwarding Engine's internal sensor table to validate the paths in a subscribe or unsubscribe request. Using these sources, Network Agent responds back to the collector with system-accepted paths and completes basic checks before proceeding to commit the request. 
•  Network Agent performs a single commit per subscribe or unsubscribe request instead of doing commits for each resource path in a request.