Fix race condition in LUGroupAssignNodes

The original code would get all node information and their groups
without before acquiring the necessary locks. With this patch the node
information is only retrieved once all locks have been acquired. Groups
are locked optimistically and verified after acquiring the node locks.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>