Timestamp as Unix timestamp

[tianyangj]

We currently store timestamps in local timezone instead of UTC. I understand it would be difficult to update all of them in UTC considering the amount of data we have. However, we can do some datetime transformation on the API, e.g., in the Entity to map out correct unix timestamp? Also, should we use unix timestamp instead of ISO 8601 format? If we use unix timestamp, it would be a loss transformation since EOS produce 2 blocks every second and unix timestamp is in second. It would be better to return ISO 8601 format like EOS chain API (they should have "Z" appended at the end, which I think is a bug on EOSIO plugin).