Change cgroups::assign to assign all threads.

Use cgroups.proc (rather than tasks) so all threads in the pid's
threadgroup are assigned.

Documentation/cgroups/cgroups.txt:
"You can use the cgroup.procs file instead of the tasks file to move all
threads in a threadgroup at once. "

Added a test to verify all threads are assigned.

This obviates the recently introduced assignAllThreads and thus it has
been removed.

Review: https://reviews.apache.org/r/19951