- add a reference count and flags to struct expand_node

- during expansion, no longer create a new node for each result but try to
  lookup for an existing equivalent node and increment its reference count
  so that: a) we save on memory, b) we don't need to expand the same users
  again and again just because they keep appearing in expansion results.
- while expanding, flag nodes as F_EXPAND_DONE so that we know which nodes
  we already processed
- be smarter when expanding, if we have a clue that an iteration has not
  brought any new result (because no new nodes were added and all existing
  nodes have F_EXPAND_DONE), end expansion and proceed to delivery.
- various small cleanups

discussed with jacekm@ yesterday, rebuild aliases db, make clean

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