mdmon: periodically checkpoint recovery

mdmon: periodically checkpoint recovery

The kernel updates and notifies md/sync_completed when it is time to
take a checkpoint.  When this occurs (at 1/16 array size intervals)
write 'idle' to md/sync_action to have the current recovery position
updated in recovery_start and resync_start.

Requires the metadata handler to reset ->last_checkpoint when it has
determined that recovery has ended.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>