Use Timestamp for timestamp in gRPC

[huan]

According to https://cloud.google.com/apis/design/naming_convention#method_names

To represent a point in time independent of any time zone or calendar, google.protobuf.Timestamp should be used, and the field name should end with time, such as start_time and end_time.

Currently, we are using uint64. Need to follow the Google Naming Convention.

See also: API style guidelines from Envy Proxy Data Plane API

For time-related fields, prefer using the well-known types google.protobuf.Duration or google.protobuf.Timestamp instead of raw integers for seconds.