When compiling with --enable-cassert, check for reference count leaks

When compiling with --enable-cassert, check for reference count leaks
in the relcache.  It's rather silly that we have reference count leak
checks in bufmgr and in catcache, but not in relcache which will normally
have many fewer entries.  Chris K-L would have caught at least one bug
in his recent DROP patch if he'd had this.