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)