Update to LocalDateTime instead of the old Date API

[dstr89]

In my Unit tests I would like to use something like this to fix the time when validating JWT:

Instant.now(Clock.fixed(
                Instant.parse("2018-08-22T10:00:00Z"),
                ZoneOffset.UTC));

But this is not possible since JJWT uses the old date API. In DefaultClock now() method.

BR,
Daniel

[lhazlewood]

Closing as a duplicate of #235 . In the meantime, you'll need to convert the JDK 8+ time concepts into a java.util.Date. HTH!