Update core timestamp to string to match other date fields

[btkostner]

Updates the core bottle timestamp to a string for ISO datetime like all the other datetime fields we have.

This will need a change to the elixir branch as well.

[doomspork]

Let's hold off on this as it doesn't really add value but does case a pretty good breaking change.

The original justification for the ISO was that the database stores DateTime, not timestamps, so we do a bunch of conversion. The message envelope doesn't need to be human readable and using timestamps makes certain things at that level of the process simpler.