Patch 1/2 -- pathsplitter

Some RumorMill changes in the janitor (and very minor related changes to RumorMill and SearchRunner).

Basically we now have 3 mills:
janitor->rumorMill -- the externally accessible mill. Middle priority. Is filled outside the janitor with either our direct peers or useful search results. I've never seen it come close to full, and it empties quickly.
janitor->splitMill -- the high priority mill. If there's a node which has known non-one-hop links, we add their getPeers responses to this mill, in an attempt to split the links. In theory it could become full immediately after a global maintenance, so the maximum size was intentionally set to be fairly low. It empties quickly.
janitor->idleMill -- the low priority mill, which is used the overwhelming majority of the time. This one tends to constantly overflow with new nodes and never empties.

Doing this causes the NodeStore to fill with keyspace neighbors (to the point that it's actually a potential problem--we keep too many of our immediate keyspace neighbors, and potentially open up holes in areas of keyspace that are only close-ish to us). This is addressed in patch 2.