Port Volkers deadlock avoidance patch to HEAD.

Port Volkers deadlock avoidance patch to HEAD.
This patch ensures that we lock all non-notify related databases first and
then the notify databases to avoiud a deadlock where samba needs to lock records on two databases at once (and notify being the second database).

Newer versions of samba would instead use the set-db-prio control to set this explicitely on a database per database basis instead of relying on  hardcoded database names. This patch will be reverted in the future when all updated versions of samba has been pushed out.