LU-14508 lfs: make mirror operations preserve timestamps

LU-14508 lfs: make mirror operations preserve timestamps

Save and try to restore the file timestamps around the various mirror
operations. Add sanity-flr tests 61[abc] to verify this.

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: I5ef754e46cfbe82c731a709209576bbfcc73af3d
Reviewed-on: https://review.whamcloud.com/42009
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>