Faster path to check an IP address against known networks

[bboreham]

We modify the critbitgo library to skip creating a route object we don't use.

The weaveworks-local modification can be removed if k-sone/critbitgo#7 is merged.

Benchmarks (fastest of 10 runs; timings are highly variable):

benchmark                         old ns/op     new ns/op     delta
BenchmarkRenderList-2             593307328     555257029     -6.41%
BenchmarkRenderHosts-2            433287444     394693695     -8.91%
BenchmarkRenderControllers-2      361384179     323186742     -10.57%
BenchmarkRenderPods-2             346144553     329953227     -4.68%
BenchmarkRenderContainers-2       201489916     200237356     -0.62%
BenchmarkRenderProcesses-2        135127597     123572216     -8.55%
BenchmarkRenderProcessNames-2     152146655     149658203     -1.64%

benchmark                         old allocs     new allocs     delta
BenchmarkRenderList-2             798184         610624         -23.50%
BenchmarkRenderHosts-2            603982         413645         -31.51%
BenchmarkRenderControllers-2      428532         284927         -33.51%
BenchmarkRenderPods-2             383885         240848         -37.26%
BenchmarkRenderContainers-2       253945         158539         -37.57%
BenchmarkRenderProcesses-2        119584         71915          -39.86%
BenchmarkRenderProcessNames-2     164045         116223         -29.15%

benchmark                         old bytes     new bytes     delta
BenchmarkRenderList-2             91234788      86609764      -5.07%
BenchmarkRenderHosts-2            65662378      60745008      -7.49%
BenchmarkRenderControllers-2      47002552      43220707      -8.05%
BenchmarkRenderPods-2             41884588      38159420      -8.89%
BenchmarkRenderContainers-2       28806560      26319518      -8.63%
BenchmarkRenderProcesses-2        13985787      12745817      -8.87%
BenchmarkRenderProcessNames-2     18001992      16745256      -6.98%