code cleanup of timestamp code

I am working on some beginner level patches to help clean up the
timestamp code in PostgreSQL.

Basically, my first few patches are aimed at removing the dependence
on the HAVE_INT_TIMESTAMP to choose types for variables. I will
eventually try to remove the use of HAVE_INT64_TIMESTAMP to choose
procedural code as well. I think that it will make the code easier to
read.

As a result, I have a few questions about the timestamp code. In what
instances is the floating point timestamp recommended? I see that
Debian and Ubuntu ship packages that use the int64 timestamps. Is the
backend smart enough to not load and use a database with timestamp
fields created with the representation not compiled into the compiler?
And finally, would this work be welcome in PostgreSQL? I view is as a
kind of beginner janitor type of project to get my feet wet.

Thanks,
wt