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.

Lustre-change: https://review.whamcloud.com/51852
Lustre-commit: TBD (from 81498f782e7c31e6e950352f4dbb2aa6f8052131)

Test-Parameters: trivial
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I58cf327d235448e93fa2ed63cefdf4dd01306e71
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/51896
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>