process directories in parallel in ChangesDirs

This commit lowers the total time spent in ChangesDirs to half during a
commit.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Move cgroups package into libcontainer
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #5772 from unclejack/improve_commit_performance

process directories in parallel in ChangesDirs
Remove the cgroups maintainer file

We don't need this because it is covered by the libcontainer MAINTAINERS
file
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #5809 from crosbymichael/move-cgroups

Move cgroups package into libcontainer