Fix splitmirror in cluster having different DM/LVM views of storage.

Fix splitmirror in cluster having different DM/LVM views of storage.
This patch also does some clean-up of the splitmirrors code.

I've attempted to clean-up the splitmirrors code to make it easier to
understand with fewer operations.  I've tried to reduce the number of
metadata operations without compromising the intermediate stages which
are necessary for easy clean-up in the even of failure.

These changes now correctly handle cluster situations - including exclusive
cluster mirrors.  Whereas before, a splitmirror operation would result in
remote nodes having LVM commands report the newly split LV with a proper
name while DM commands would report the old (pre-split) names of the device.
IOW, there was a kernel/userspace mismatch.