Timestamp precision and byte representation should be consistent across SDKs

Runners cannot have a timestamp precision that is lesser than what a SDK uses. Also with Fn API a runner should be able to support multiple SDKs that maybe written in different languages. It is important that we standardize the precision of timestamp to ensure that runners are compatible across all the SDKs.

NOTE: Timestamps are shifted such that lexicographic ordering of the bytes corresponds to chronological order. This also needs to be consistent across SDKs.