core: remove read lock of template being exported

Replace the exclusive lock which was taken for a template that is being
exported and the lock status in the DB, with persistent shared lock for
the template on the data domain plus persistent exclusive lock for the
template on the export domain.

Change-Id: I830cfef82063d2dbac3689a46758090da8b10df3
Bug-Url: https://bugzilla.redhat.com/920150
Signed-off-by: Arik Hadas <ahadas@redhat.com>