LU-11025 dne: refactor dir migration

LU-11025 dne: refactor dir migration

Tidy up directory migration to make it easier to support
directory restripe.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Ibad46d8cf8f984e5f0062792c651b9753c2fb9a6
Reviewed-on: https://review.whamcloud.com/37712
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>