s4:python Push some helper functions from SamDB into samba.Ldb

s4:python Push some helper functions from SamDB into samba.Ldb

This makes it possible to do a bit more of the provision with Samba
helpers, but without some of the otherwise useful things (such as
loading in the global schema) that SamDB does.

Rewrite provision_erase to use a recursive search, rather than a
looping subtree search.  This is much more efficient, particularly now
we have one-level indexes enabled.

Delete the @INDEX and similar records *after* deleting all other
visible records, this hopefully also assists performance.

Andrew Bartlett