pgsql: Adjust creation/destruction of TupleDesc data structure to reduce

Log Message:
-----------
Adjust creation/destruction of TupleDesc data structure to reduce the
number of palloc calls. This has a salutory impact on plpgsql operations
with record variables (which create and destroy tupdescs constantly)
and probably helps a bit in some other cases too.

Modified Files:
--------------
pgsql/src/backend/access/common:
tupdesc.c (r1.108 -> r1.109)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/common/tupdesc.c.diff?r1=1.108&r2=1.109)
pgsql/src/backend/catalog:
index.c (r1.245 -> r1.246)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/index.c.diff?r1=1.245&r2=1.246)
pgsql/src/backend/utils/cache:
relcache.c (r1.215 -> r1.216)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/cache/relcache.c.diff?r1=1.215&r2=1.216)
pgsql/src/include/access:
tupdesc.h (r1.46 -> r1.47)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/tupdesc.h.diff?r1=1.46&r2=1.47)