Serialize Instant to BSON datetime when possible

[frost13it]

For now, only java.util.Date can be serialized to native BSON datetime. This PR makes Instant serializable in this format as well.
But doing so can lead to a loss of precision, so it is performed only when SerializationFeature.WRITE_DATE_TIMESTAMPS_AS_NANOSECONDS is disabled. Otherwise Instant is serialized the same way as before.

[rlodge]

Sorry this has taken so long for me to get to.

I think this is a great feature to have, thanks for putting it together. I wanted to require that it be affirmatively enabled, rather than just relying on WRITE_DATE_TIMESTAMPS_AS_NANOSECONDS. I also wanted to avoid having to modify DBEncoderBsonGenerator because getting the configuration into that location isn't simple. So I tweaked this in a couple of ways and implemented without pulling in this PR.

I'm going to close this, please take a look at the implementation I put in, and if you need something else changed submit a new PR.