Clean up non-reentrant interface for hash_seq/HashTableWalk, so that

Clean up non-reentrant interface for hash_seq/HashTableWalk, so that
starting a new hashtable search no longer clobbers any other search
active anywhere in the system.  Fix RelationCacheInvalidate() so that
it will not crash or go into an infinite loop if invoked recursively,
as for example by a second SI Reset message arriving while we are still
processing a prior one.