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.