s4-drepl: Block GetNCChanges during a DsReplicaSync

s4-drepl: Block GetNCChanges during a DsReplicaSync

If we do not block these, we can get RPC faults
(DCERPC_NCA_S_PROTO_ERROR) which gives WERR_WRITE_FAULT back to the
DsReplicaSync call as there are two outstanding requests on the wire
at the one time.

We will get to the next operation as soon as this is finished
when we call run_pending_ops().

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12926

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Sun Jul 23 12:32:49 CEST 2017 on sn-devel-144