LU-10916 lfs: improve lfs mirror resync

LU-10916 lfs: improve lfs mirror resync

Make mirror resync use read+write+write+... mode instead do the
resync on each stale mirror of a file separately (read+write,
read+write, ...).

Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: I627fa53fcfde4811b2cd9c84c8545defe151206c
Reviewed-on: https://review.whamcloud.com/32808
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>