replmd: RMD_VERSION incorrectly incremented for link conflicts

replmd: RMD_VERSION incorrectly incremented for link conflicts

This problem was noticed when 2 DCs added the same linked attribute at
roughly the same time. One DC would have a later timestamp than the
other, so it would re-apply the same link information. However, when it
did this, replmd_update_la_val() would incorrectly increment the
RMD_VERSION for the attribute. We then end up with one DC having a
higher RMD_VERSION than the others (and it doesn't replicate the new
RMD_VERSION out).

During replication RMD_VERSION is used to determine whether a linked
attribute is old (and should be ignored), or whether the information is
new and should be applied to the DB. This RMD_VERSION discrepancy could
potentially cause a subsequent linked attribute update to be ignored.

Normally when a local DB operation is performed, we just pass in a
version of zero and get replmd_update_la_val() to increment what's
already in the DB. However, we *never* want this to happen during
replication - we should always use the version we receive from the peer
DC.

This patch fixes the problem by separating the API into two:
- replmd_update_la_val(): we're updating a linked attribute in the DB,
  and so as part of this operation we always want to increment the
  version number (the version no longer need to be passed in because
  we can work it out from the existing DB entry).
- replmd_set_la_val(): we want to set a linked attribute to use the
  exact values we're telling it, including the version. This is what
  replication needs to use.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13038
Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Tue Sep 26 09:36:48 CEST 2017 on sn-devel-144