Improve docker clientside net setup: hold global lock less.
Prior to this commit, I had reused all the libvnode:: network functions, but that requires these system-wide indexes to be built (i.e. bridges, ifaces) before using the helpers -- and on any network state change. Those indexing functions take a long time on heavily-loaded systems (i.e., 5k processes, hundreds of ifaces). The helpers become very fast; but for the case of large numbers of vnodes on one vhost, it is better to hold the global lock for less time, and pay a higher per-helper cost, instead of making use of a nicely-indexed cache. So now we cache nothing and (hopefully) use /sys intelligently to avoid forking unnecessarily.
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