LU-6785 utils: compatibility fix for lfs migrate

LU-6785 utils: compatibility fix for lfs migrate

Make lfs migrate able to operate against servers that support
file lease but not atomic swap/close (as added in LU-4840).

Make blocking mode the default and add a --non-block switch to
lfs accordingly.

Signed-off-by: Henri Doreau <henri.doreau@cea.fr>
Change-Id: I7d3cdccfcc5bec78e69c69615ee29cac11f6f31f
Reviewed-on: http://review.whamcloud.com/16238
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>