- Make hash sizes growable, to satisfy users running large mpd
installations, having thousands of nodes.
- NG_NAMEHASH() proved to give a very bad distribution in real life
name sets, while generic hash32_str(name, HASHINIT) proved to give
an even one, so you the latter for name hash.
- Do not store unnamed nodes in slot 0 of name hash, no reason for that.
- Use the ID hash in cases when we need to run through all nodes: the
NGM_LISTNODES command and in the vnet_netgraph_uninit().
- Implement NGM_LISTNODES and NGM_LISTNAMES as separate code, the former
iterates through the ID hash, and the latter through the name hash.
- Keep count of all nodes and of named nodes, so that we don't need
to count nodes in NGM_LISTNODES and NGM_LISTNAMES. The counters are
also used to estimate whether we need to grow hashes.
- Close a race between two threads running ng_name_node() assigning same
name to different nodes.
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