Container performance improvements

[conorbranagan]

Improves the performance of the container checks significantly:

(from a test w/ 70 running containers, 10s cache)

benchmark                    old ns/op     new ns/op     delta
BenchmarkAllContainers-2     78556378      3733144       -95.25%

benchmark                    old allocs     new allocs     delta
BenchmarkAllContainers-2     38768          6330           -83.67%

benchmark                    old bytes     new bytes     delta
BenchmarkAllContainers-2     7538615       1143256       -84.83%

Changes included:

• Removing stats we don't use from CPU and Memory to cut down on file reads. Caching MemLimit and CPULimit after the first run. (dca5876)
• Adding caching to container list metadata so we only get the latest containers every 10s. (0505470)
• General alloc optimizations in file reading, etc. (e44b0de)
• Parsing the docker health from the container list API. This allows us to remove the CollectContainerHealth option and have it on by default, because there is no perf impact. (596537b)

[shang-wang]

LGTM, great improvement too