LU-17814 utils: implement thread pool

LU-17814 utils: implement thread pool

Implement thread pool for parallel find - still uses
regular find code to do the actual find.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: If5ebd3b52b93fc54dd4ab86bd8dfe06c4dcc0c11
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57292
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Marc Vef <mvef@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>