pgsql: Refactor ExecGetJunkAttribute to avoid searching for junk

Log Message:
-----------
Refactor ExecGetJunkAttribute to avoid searching for junk attributes
by name on each and every row processed. Profiling suggests this may
buy a percent or two for simple UPDATE scenarios, which isn't huge,
but when it's so easy to get ...

Modified Files:
--------------
pgsql/src/backend/executor:
execJunk.c (r1.54 -> r1.55)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execJunk.c.diff?r1=1.54&r2=1.55)
execMain.c (r1.280 -> r1.281)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execMain.c.diff?r1=1.280&r2=1.281)
pgsql/src/include/executor:
executor.h (r1.130 -> r1.131)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/executor/executor.h.diff?r1=1.130&r2=1.131)
pgsql/src/include/nodes:
execnodes.h (r1.161 -> r1.162)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/execnodes.h.diff?r1=1.161&r2=1.162)