s4-dsdb: Initialize module context only we are to handle Tombstone request

Signed-off-by: Kamen Mazdrashki <kamenim@samba.org>
s4-dsdb: Make most specific objectCategory for an object

This is lightweight implementation and should be used on objects
with already verified objectClass attribute value - eg. valid classes,
sorted properly, etc.
Checkout objectclass.c module for heavy weight implementation.

Signed-off-by: Kamen Mazdrashki <kamenim@samba.org>