tdb: enforce hashing, via example hash in old rwlocks entry in header.

tdb: enforce hashing, via example hash in old rwlocks entry in header.

This means that older code will not be able to open new TDBs with
a non-default hash, *even* if they are using the correct hash.
Non-default hashes were unusual, but now SAMBA is considering using
a non-default hash, and avoiding corruption seems more important
than backwards compatibility for an obscure case.