Make Orient use joda DateTime for OType.DATETIME fields

When getting/setting date fields in Orient, we currently use java.util.Date and convert to DateTime in our own code. For example:

https://github.com/sonatype/nexus-oss/commit/4cf774b9483f96463f19c2a493507d5ae08516a8#diff-bf7a86115b1c6f55a819dd6beb26e5a6R68

Instead of this, we'd like to register a serializer/deserializer for DateTime with Orient so we can just use them directly.

The goal of this improvement is to either find a way to do it using the OrientDB api, or submit and follow through with a request to make this kind of thing possible with OrientDB, and change our code to use it and then consistently just use DateTime directly.

This kind of capability existed at one point for orient's object API, as evidenced by this code snippet I found, but it's unclear whether it's ever been possible for the document or graph apis:
https://github.com/dozed/orientdb-test/blob/master/src/main/java/org/noorg/orientdb/test/Database.java#L63