Databricks: Support Timetravel With "TIMESTAMP AS OF"

[JamesVorder]

This PR adds support for Databricks' timstamp-based time travel. This is a first step toward resolvign #2006.

The following is now supported:

SELECT 1 FROM t1 TIMESTAMP AS OF '2018-10-18T22:15:12.013Z';
SELECT 1 FROM t1 TIMESTAMP AS OF CURRENT_TIMESTAMP() - INTERVAL 12 HOURS;
-- etc.

[iffyio]

@JamesVorder could you take a look at the ci failure?

[JamesVorder]

@iffyio Can you see if the build failure is resolved? Looks like it was just a silly mistake with a docstring.

[iffyio]

LGTM! Thanks @JamesVorder!