cmd/containerd-shim: aggressive memory reclamation
cmd/containerd-shim: aggressive memory reclamation

To avoid having the shim hold on to too much memory, we've made a few
adjustments to favor more aggressive reclamation of memory from the
operating system. Typically, this would be negligible, on the order of a
few megabytes, but this is impactful when running several containers.

The first fix is to lower the threshold used to determine when to run
the garbage collector. The second runs `runtime/debug.FreeOSMemory` at a
regular interval.

Under test, this result in a sustained memory usage of around 3.7 MB.

Signed-off-by: Stephen J Day <63efd86fa22882ae89b6027b22a36e4db1f4214d@docker.com>

crosbymichael/containerd,miaoyq/containerd,estesp/containerd,kunalkushwaha/containerd,estesp/containerd,containerd/containerd,miaoyq/containerd,dmcgowan/containerd,containerd/containerd,vdemeester/containerd,kunalkushwaha/containerd,vdemeester/containerd,dmcgowan/containerd,thaJeztah/containerd,mikebrow/containerd,mikebrow/containerd,crosbymichael/containerd,thaJeztah/containerd