pgsql: Change the timestamps recorded in transaction commit/abort xlog

Log Message:
-----------
Change the timestamps recorded in transaction commit/abort xlog records
from time_t to TimestampTz representation. This provides full gettimeofday()
resolution of the timestamps, which might be useful when attempting to
do point-in-time recovery --- previously it was not possible to specify
the stop point with sub-second resolution. But mostly this is to get
rid of TimestampTz-to-time_t conversion overhead during commit. Per my
proposal of a day or two back.

Modified Files:
--------------
pgsql/src/backend/access/transam:
twophase.c (r1.29 -> r1.30)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/twophase.c.diff?r1=1.29&r2=1.30)
xact.c (r1.241 -> r1.242)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c.diff?r1=1.241&r2=1.242)
xlog.c (r1.267 -> r1.268)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.267&r2=1.268)
pgsql/src/backend/utils/adt:
timestamp.c (r1.175 -> r1.176)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/timestamp.c.diff?r1=1.175&r2=1.176)
pgsql/src/include/access:
xact.h (r1.86 -> r1.87)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xact.h.diff?r1=1.86&r2=1.87)
xlog_internal.h (r1.19 -> r1.20)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xlog_internal.h.diff?r1=1.19&r2=1.20)
pgsql/src/include/utils:
timestamp.h (r1.68 -> r1.69)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/timestamp.h.diff?r1=1.68&r2=1.69)