Disallow jsonpath methods involving TZ in immutable functions

Disallow jsonpath methods involving TZ in immutable functions

Timezones are not immutable and so neither is any function that relies on
them. In commit 66ea94e8, we introduced a few methods which do casting
from one time to another and thus may involve the current timezone.  To
preserve the immutability of jsonpath functions currently marked
immutable, disallow these methods from being called from non-TZ aware
functions.

Jeevan Chalke, per a report from Jian He.