LU-11245 flr: lfs mirror dump command

LU-11245 flr: lfs mirror dump command

Adds "lfs mirror dump" command to dump a mirror's content of a
mirrored file.

Usage:

lfs mirror dump <-N|--mirror-id <mirror_id>>
	[-o|--outfile <output_file> ] <mirrored_file>

Options:

--mirror-id <mirror_id>
  Specify the mirror by mirror_id to dump its content.
--outfile <output_file>
  Dump the content into file output_file, if not specified, the
  content will be dump to the standard output stream.

Test-Parameters: testlist=sanity-flr
Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: I2846b1c1968d9cc302c07b57a05687af50530d53
Reviewed-on: https://review.whamcloud.com/32997
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@gmail.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>