6. core: Persist all async task placeholders

Add the infra structure code for persisiting
all async task placeholders for all child commands
before submitting any task to vdsm

http://www.ovirt.org/Features/AsyncTaskManagerChanges_3.3
Change-Id: I66628b98298ed1f500946889604ad906f0a68022
Signed-off-by: Ravi Nori <rnori@redhat.com>
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>