Fix KPI_Event2 timestamp value

KPI_Event2 timestamps are suppose to be in seconds since epoch (float64).   OpenOLT adapter and voltha-lib-go are storing the nanoseconds since the epoch (int64) in the float64 field.

Correct the message format on KAFKA to be in seconds since epoch.

The internal use of the timestamp as parameters passed to functions or internally can remain in int64 nanoseconds to minimize code impact, so only converting to float64 seconds for the protobuf is needed.

Reference: ts field at line 40 of MetricMetaData message in events.proto 

Reference: ts field at line 103 of KpiEvent2 message in events.proto