r2051: switched the samdb over to using the new destructor and reference

r2051: switched the samdb over to using the new destructor and reference
count features of talloc, instead of re-implementing both those
features inside of samdb (which is what we did before).

This makes samdb considerably simpler, and also fixes some bugs, as I
found some error paths that didn't call samdb_close(). Those are now
handled by the fact that a talloc_free() will auto-close and destroy
the samdb context, using a destructor.
(This used to be commit da60987a92266734c33b81ee217081abdc4330f3)