Fixed patch for timestamp refactor effort

Here is a fixed patch for the timestamp refactor. I apologize for sending the
wrong patch earlier.

I would like to get some comments on if this patch is heading in the right
direction. Here's the proposal for what I am doing:

Proposal for Refactoring of Timestamp Datatype

Goal:
The primary goal of this effort is to remove as many of the #ifdef tests for
HAVE_INT64_TIMESTAMP from timestamp.c[1] as possible. I have played with
PostgreSQL for many years and I look at this as an opportunity to give back to
the community.

Plan:
1. Replace datatypes selected by value of HAVE_INT64_TIMESTAMP with typedefs.
The typedefs will most likely be placed in timestamp.h[2].
2. Replace the places where the logic is determined by the value of
HAVE_INT64_TIMESTAMP with macros or function calls.

Further Investigation:
It may also be useful to investigate if PostgreSQL still needs two different
representations for timestamps (int64 or double).

[1]$(srcdir)/src/backend/utils/adt/timestamp.c
[2]$(srcdir)/src/include/utils/timestamp.h

Thanks,
wt