Date field sorting

[maheshrajamani]

What this PR does:
Added sorting for date field.

Which issue(s) this PR fixes:
Fixes #374

Checklist

• Changes manually tested
• Automated Tests added/updated
• Documentation added/updated
• CLA Signed: DataStax CLA

[tatu-at-datastax]

Mostly looks good, but I think this is missing changes to JsonNodeComparator; it would be counting on sorting by Numbers, overlapping with Number type vs Date; and I think this is not fully correct. We may need to discuss this further.
It sort of kind of works, for actual usage. But I think Mongo would separate Date values from Numbers (would not mix entries with column values of Date and Number types).

I know that our in-memory sorting is expected to be temporary, and replaced by global sort so this may be acceptable.
But depends on how closely we want to implement Mongo-compatible logic.
I think for most users sorting numerically would work ok tho; mixing Numbers and Dates is probably not a common use case (or other mixing; precedence of Date vs other types is different from that of Number)

Now: to fully implement this, JsonNodeComparator would I think need to auto-detect Date from JsonNode.

[maheshrajamani]

Now: to fully implement this, JsonNodeComparator would I think need to auto-detect Date from JsonNode.

As per discussion with @tatu-at-datastax, have used PojoNode for Date type.

[tatu-at-datastax]

Now: to fully implement this, JsonNodeComparator would I think need to auto-detect Date from JsonNode.

As per discussion with @tatu-at-datastax, have used PojoNode for Date type.

This works beautifully since it can be reliably created from query_timestamp_values (no need for EJSON encoding) and sorts exactly as per spec. Good job!

[vkarpov15]

I'd recommend adding another query to this test that sorts in reverse order, and asserts that sorting by new FindOperation.OrderBy("sort_date", false) returns doc6, doc5, doc4, doc3, doc2. This test as written might succeed even if no sorting was done, because the documents happen to be in ascending order by sort_date.

[vkarpov15]

One minor comment but LGTM overall