Allow paging to disk before oom killer runs

Allow 25% swap space over and above the assigned memory limit for a
Warden container. The oom killer will now run only when either:

(a) memory usage exceeds the limit and no further pages can be swapped
    out or otherwise reclaimed,

or:

(b) memory usage plus swap space exceeds the memory limit by over 25%.

So, if memory usage hits the memory limit and swapping is necessary to
bring memory usage back below the limit and swap space is available,
the cgroup's memory will start paging to disk until the swap space
exceeds the memory limit by 25%, at which point the oom killer will
run.

Note: commit aa6cc0c set the above limits to be equal purportedly in
order to prevent the container from swapping. It is not clear why the
limits being equal would prevent swapping in all circumstances, e.g.
if there was pressure on memory other than from the memory resource
controller. Also no rationale was given for attempting to prevent
swapping (and no issue number provided which might have supplied the
rationale). Before aa6cc0c, swapping was unbounded which could give
rise to poor performance, so the current commit does better than
simply reverting aa6cc0c.
Ignore IDEA .iml files