Send event as JsonIetfVal instead of StringVal

[renukamanavalan]

Minor additional changes:

1. Skip test event used by Switch's monit code
2. qsize between telemetry & gNMI client is configurable.

gnmi_cli -client_types=gnmi -a 127.0.0.1:50051 -t EVENTS -logtostderr -insecure -v 7 -streaming_type ON_CHANGE -q all[heartbeat=5][qsize=1000] -qt s

Sets heartbeat to every 5 seconds
Set qsize to be 1000

Both params are optional.
Heartbeat default: Every 2 seconds
Qsize default: 10240.

To test qsize, look for the following log lines in /var/log/telemetry.log

Oct  4 21:06:44.062902 str-s6000-on-5 INFO telemetry#supervisord: telemetry I1004 21:06:44.058791      15 events_client.go:97] Events priority Q max set default = 10240
Oct  4 21:06:44.071703 str-s6000-on-5 INFO telemetry#supervisord: telemetry I1004 21:06:44.066958      15 events_client.go:115] Events priority Q max set by qsize param = 1000