[TO_STAGE] Bug 1171289 - background pkill command in OOMPlugin

There are two significant fixes here:

1) Instead of using ApplicationContainer.kill_procs, which waits for pkill/pgrep process to return, we simply spawn a pkill and detach it, because we don't actually care about its return code, and we cannot wait for it.

2) The while loop runs when oom_control['under_oom'] is 1, even if the memory usage indicates that the cgroup has fallen below its memsw limit.

It won't create zombies -- that's what the detach on the next line is for.  Will it possibly hang for a long time?  yes, that's the chicken-and-egg problem we have here.  It will hang until the gear has enough memory to run long enough to release locks on tasks structures that pkill is waiting for.