Release unneeded locks while replacing disks

If an iallocator is used, “gnt-instance replace-disks” would acquire the
locks of all nodes (only the allocator will decide which node to use).
Unfortunately the unneeded locks were not released during the operation,
causing unnecessary delays for other jobs.

This patch changes the LU to release unneeded locks and adds assertions.

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