Clean up API for timestamp formatting in JSON encoder and text encoder

[prashantv]

See #123 (comment) for context.

Currently, we have EpochFormatter, NoTime, RFC3339Formatter, TextNoTime, and TextTimeFormat.

We can create a separate option for setting the timestamp key in the JSON encoder, and share the time formatters between the JSON encoder and the text encoder.

[akshayjshah]

Landed on dev as part of #264.