Implement precision support for timestamp and time, both with and without

Implement precision support for timestamp and time, both with and without
 time zones.
SQL99 spec requires a default of zero (round to seconds) which is set
 in gram.y as typmod is set in the parse tree. We *could* change to a
 default of either 6 (for internal compatibility with previous versions)
 or 2 (for external compatibility with previous versions).
Evaluate entries in pg_proc wrt the iscachable attribute for timestamp and
 other date/time types. Try to recognize cases where side effects like the
 current time zone setting may have an effect on results to decide whether
 something is cachable or not.