Modify APIs to make the network setup async

Based on discussions we had on decreasing container/pod creation time by doing the network setup in parallel - we can think of callback/ or any async based approach for setting up networks.

cc @crosbymichael

No files or tests are named. Start by reviewing the existing Go APIs for network setup and the discussion about container/pod creation time, then determine how asynchronous completion and parallel network setup should be represented; the work is done when the API supports that behavior with documented completion semantics.

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