Modify AtEOXact_CatCache and AtEOXact_RelationCache to assume that the

ResourceOwner mechanism already released all reference counts for the
cache entries; therefore, we do not need to scan the catcache or relcache
at transaction end, unless we want to do it as a debugging crosscheck.
Do the crosscheck only in Assert mode.  This is the same logic we had
previously installed in AtEOXact_Buffers to avoid overhead with large
numbers of shared buffers.  I thought it'd be a good idea to do it here
too, in view of Kari Lavikka's recent report showing a real-world case
where AtEOXact_CatCache is taking a significant fraction of runtime.