pgsql: Get rid of a bunch of #ifdef HAVE_INT64_TIMESTAMP conditionals by

Log Message:
-----------
Get rid of a bunch of #ifdef HAVE_INT64_TIMESTAMP conditionals by inventing
a new typedef TimeOffset to represent an intermediate time value.  It's
either int64 or double as appropriate, and in most usages will be measured
in microseconds or seconds the same as Timestamp.  We don't call it
Timestamp, though, since the value doesn't necessarily represent an absolute
time instant.

Warren Turkal

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        date.c (r1.139 -> r1.140)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/date.c?r1=1.139&r2=1.140)
        nabstime.c (r1.153 -> r1.154)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/nabstime.c?r1=1.153&r2=1.154)
        timestamp.c (r1.185 -> r1.186)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/timestamp.c?r1=1.185&r2=1.186)
    pgsql/src/include/utils:
        date.h (r1.39 -> r1.40)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/date.h?r1=1.39&r2=1.40)
        timestamp.h (r1.75 -> r1.76)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/timestamp.h?r1=1.75&r2=1.76)