Add first-class encoder support for times and durations

[akshayjshah]

Remove the Time field constructor in favor of more-explicit Epoch (floating-point seconds since epoch) and ISO8601 constructors.

If necessary, we can also include EpochMillis (int64 milliseconds since epoch).

[jcorbin]

I find going this route a little unfortunate: I'd rather stay with Time and get the encoders looped in "what does it mean to encode time", rather than have it variable per-field-per-log-site.

At any rate, an EpochMillis should probably be float64 rather than int64.

[akshayjshah]

That's a fair point, particularly for concepts so intimately connected with logging. I'll amend the title of this issue to reflect that.

[akshayjshah]

Landed on dev.