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 <stephen.day@docker.com>
0e8f08476c