Improve ResourceOwners' behavior for large numbers of owned objects.

The original coding was quite fast so long as objects were always
released in reverse order of addition; otherwise, it degenerated into
O(N^2) behavior due to searching for the array element to delete.
Improve matters by switching to hashed storage when the number of
objects of a given type exceeds 64.  (The cutover point is open to
discussion, of course, but some simple performance testing suggests
that hashing has enough overhead to be a loser below there.)

Also, refactor resowner.c so that we don't need N copies of the array
management code.  Since all the resource IDs the code currently needs
to deal with are either pointers or integers, it seems sufficient to
create a one-size-fits-all infrastructure in which everything is
converted to a Datum for storage.

Aleksander Alekseev, reviewed by Stas Kelvich, further fixes by me

src/backend/access/hash/hashfunc.c

@@ -297,6 +297,9 @@ hashvarlena(PG_FUNCTION_ARGS)
  * of 2.  There is no need to do mod a prime (mod is sooo slow!).
  * If you need less than 32 bits, use a bitmask.
  *
+ * This procedure must never throw elog(ERROR); the ResourceOwner code
+ * relies on this not to fail.
+ *
  * Note: we could easily change this function to return a 64-bit hash value
  * by using the final values of both b and c.  b is perhaps a little less
  * well mixed than c, however.