pgsql: Arrange to fsync two-phase-commit state files only during

Log Message:
-----------
Arrange to fsync two-phase-commit state files only during checkpoints;
given reasonably short lifespans for prepared transactions, this should
mean that only a small minority of state files ever need to be fsynced
at all. Per discussion with Heikki Linnakangas.

Modified Files:
--------------
pgsql/src/backend/access/transam:
twophase.c (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/twophase.c.diff?r1=1.3&r2=1.4)
xact.c (r1.206 -> r1.207)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c.diff?r1=1.206&r2=1.207)
xlog.c (r1.201 -> r1.202)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.201&r2=1.202)
pgsql/src/include/access:
twophase.h (r1.2 -> r1.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/twophase.h.diff?r1=1.2&r2=1.3)