LUNodeSetParams: Lock affected instances only

Until now LUNodeSetParams would lock all instances if a node's
secondary IP address was to be changed and would then release
all instances it didn't actually need. With this patch the LU
optimistically locks instances and, once it got the locks,
checks whether they're still correct. This is similar to how
node group locking is done.

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