Repair memory leaks that caused CacheCxt to grow without bound. We

really ought to fix relcache entry construction so that it does not
do so much with CurrentMemoryContext = CacheCxt.  As is, relatively
harmless leaks in either sequential or index scanning translate to
permanent leaks if they occur when called from relcache build.
For the moment, however, the path of least resistance is to repair
all such leaks...