s4-drs: Implementation of GetNCChanges extended op 6 - fsmo role transfer

s4-drs: Implementation of GetNCChanges extended op 6 - fsmo role transfer

Basically the candidate owner makes a getncchanges call with extended op 6 when they want to
become the new owner. The current owner then updates the corresponding fSMORoleOwner attribute
in its database with the new owner, and replicates the change to the candidate, who then becomes the
owner.
The patch was made in cooperation with Anatoliy Atanasov <anatoliy.atanasov@postpath.com> who
kindly helped to debug it.