Fix date and timestamp conversion

[rdblue]

This fixes problems with Date and Timestamp conversion:

• Dates must be converted to days from epoch, not micros
• Timestamp#toLocalDate converts with respect to the timestamp's time zone instead of UTC so the date is inaccurate
• Timestamp access was corrupting the timestamp value and discarding microseconds (HIVE-19726) so this accesses the value directly

This converts directly and fixes the tests using Iceberg literals. The timestamp test was updated to create a Timestamp to pass into Hive directly from a known value in microseconds because parsing is dependent on the local zone. That dependency is okay when Hive is running, but we need a Timestamp with a known UTC instant for the test.

[cmathiesen]

Thank you very much for helping with all the Timestamp/Date stuff, it all looks good! I think theres just one checkstyle issue with an unused import on the test class which I'll push a change for after merging this :)