Updated the cgroup listener to not fail on discard.
Updated the cgroup listener to not fail on discard.

Before, running any task would cause the following error to be logged
when the OOM listener was finalized:

E0418 14:13:25.575969 31596 memory.cpp:637] Listening on OOM events
failed for container 65e9cbb5-2e29-4cac-bc98-2723a359a652:
Event listener is terminating

The error originates in the OOM listener finalization, which will
return a failed future even if the listener was terminated due to
a discard request. The memory subsystem does such a discard during
cgroup cleanup.

This removes a lot of noise from the logs, which also makes
debugging a bit easier by avoid following red-herrings.

This closes #359

dforsyth/mesos,dforsyth/mesos,dforsyth/mesos,Gilbert88/mesos,Gilbert88/mesos,abudnik/mesos,reneploetz/mesos,abudnik/mesos,abudnik/mesos,dforsyth/mesos,abudnik/mesos,dforsyth/mesos,reneploetz/mesos,abudnik/mesos,dforsyth/mesos,dforsyth/mesos,abudnik/mesos,Gilbert88/mesos,Gilbert88/mesos,reneploetz/mesos,Gilbert88/mesos,Gilbert88/mesos,Gilbert88/mesos,reneploetz/mesos,reneploetz/mesos,abudnik/mesos,reneploetz/mesos,reneploetz/mesos