Split tagged-hash from regular history backend

[eagle]

The history backend currently uses the dbz code in lib, which builds with tagged hash depending on compile-time options. This defeats the whole point of having a pluggable history API. Tagged hash should be a separate history backend that can be selected at run-time.

This will requiring untangling the code in lib/dbz.c and updating makedbz to call into the API instead of knowing about the dbz format internally.