pgsql: Clean up the rather historically encumbered interface to now()

Log Message:
-----------
Clean up the rather historically encumbered interface to now() and
current time: provide a GetCurrentTimestamp() function that returns
current time in the form of a TimestampTz, instead of separate time_t
and microseconds fields. This is what all the callers really want
anyway, and it eliminates low-level dependencies on AbsoluteTime,
which is a deprecated datatype that will have to disappear eventually.

Modified Files:
--------------
pgsql/contrib/btree_gist:
btree_ts.c (r1.8 -> r1.9)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/btree_gist/btree_ts.c.diff?r1=1.8&r2=1.9)
pgsql/contrib/spi:
timetravel.c (r1.22 -> r1.23)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/spi/timetravel.c.diff?r1=1.22&r2=1.23)
pgsql/src/backend/access/transam:
xact.c (r1.208 -> r1.209)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c.diff?r1=1.208&r2=1.209)
xlog.c (r1.203 -> r1.204)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.203&r2=1.204)
pgsql/src/backend/bootstrap:
bootparse.y (r1.76 -> r1.77)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/bootstrap/bootparse.y.diff?r1=1.76&r2=1.77)
bootscanner.l (r1.39 -> r1.40)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/bootstrap/bootscanner.l.diff?r1=1.39&r2=1.40)
pgsql/src/backend/libpq:
crypt.c (r1.63 -> r1.64)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/crypt.c.diff?r1=1.63&r2=1.64)
pgsql/src/backend/postmaster:
pgstat.c (r1.97 -> r1.98)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c.diff?r1=1.97&r2=1.98)
postmaster.c (r1.455 -> r1.456)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c.diff?r1=1.455&r2=1.456)
pgsql/src/backend/tcop:
postgres.c (r1.450 -> r1.451)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c.diff?r1=1.450&r2=1.451)
pgsql/src/backend/utils/adt:
datetime.c (r1.150 -> r1.151)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/datetime.c.diff?r1=1.150&r2=1.151)
nabstime.c (r1.133 -> r1.134)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/nabstime.c.diff?r1=1.133&r2=1.134)
pgstatfuncs.c (r1.23 -> r1.24)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/pgstatfuncs.c.diff?r1=1.23&r2=1.24)
selfuncs.c (r1.182 -> r1.183)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/selfuncs.c.diff?r1=1.182&r2=1.183)
timestamp.c (r1.126 -> r1.127)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/timestamp.c.diff?r1=1.126&r2=1.127)
pgsql/src/include/access:
xact.h (r1.77 -> r1.78)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xact.h.diff?r1=1.77&r2=1.78)
pgsql/src/include:
pgstat.h (r1.31 -> r1.32)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pgstat.h.diff?r1=1.31&r2=1.32)
pgsql/src/include/utils:
nabstime.h (r1.45 -> r1.46)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/nabstime.h.diff?r1=1.45&r2=1.46)
timestamp.h (r1.45 -> r1.46)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/timestamp.h.diff?r1=1.45&r2=1.46)