LU-17011 utils: monotonic clock in lfs mirror

LU-17011 utils: monotonic clock in lfs mirror

use monotonic clocks instead of realtime to avoid affecting
bandwidth or hanging the transfer if the clock is changed.

Test-Parameters: trivial
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I58cf327d235448e93fa2ed63cefdf4dd01306e71
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51852
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>