Merge on activate support.

Merge on activate support.

If either the origin or snapshot that is to be merged is open the merge
will not start; only the merge metadata will be written.  The merge will
start on the next activation of the origin (or via lvchange --refresh)
IFF both the origin and snapshot are closed.

Merge on activate is particularly important if we want to merge over a
mounted filesystem that cannot be unmounted (until next boot) --- for
example root.