Big update for timestamp etc.

I've committed (most of?) the changes to implement a timestamp without
time zone type. To help with backward compatibility and upgrading, I've
made the *default* timestamp the one with time zone. That is, if you
want a timestamp without time zone you will have to fully specify it
verbatim. SQL99 calls for "without time zone" to be the default, and we
can make it so in the next release.

I have not made any changes to pg_dump or psql or any other utility or
client-side driver to recognize the new types or to do conversions to
"SQL99 names", but these should be done before we exit beta.

I can not actually *check* my changes to the cvs tree, since
cvsup.postgresql.org apparently does not yet see the changes to
cvs.postgresql.org. This is pretty annoying from my pov; hopefully the
gain in distributing the load offsets the problems in syncing from a
slave machine rather than "truth".

Details from the cvs log follow.

- Thomas

Measure the current transaction time to milliseconds.
Define a new function, GetCurrentTransactionStartTimeUsec() to get the
time
to this precision.
Allow now() and timestamp 'now' to use this higher precision result so
we now have fractional seconds in this "constant".
Add timestamp without time zone type.
Move previous timestamp type to timestamp with time zone.
Accept another ISO variant for date/time values: yyyy-mm-ddThh:mm:ss
(note the "T" separating the day from hours information).
Remove 'current' from date/time types; convert to 'now' in input.
Separate time and timetz regression tests.
Separate timestamp and timestamptz regression test.