Stop using ISO strings for timestamps in API

[EvanHahn]

We use ISO strings for timestamps. This has a few issues, all minor:

• It’s more data to (de)serialize over the IPC bridge
• It’s harder to validate
• It’s slower to parse as a Date (I assume)

To fix this, we should use something else (probably numbers) in the API.

To be clear, this does not affect any data on disk. That's stored as a protobuf google.protobuf.Timestamp, and that doesn't change as part of this task.

This probably requires work in @mapeo/schema more than anywhere else.