The no-connect net (0) should never drive the net of a cluster.
Otherwise, tracks connecting between unconnected pads and connected pads
can acquire net 0 incorrectly.
Rather than duplicating the connectivity calculations in determining
whether a track is dangling, we utilize the current database to find the
number of connected items for an anchor item when feasible.
Multiple anchors such as tracks still need additional logic.
Fixes: lp:1805479
* https://bugs.launchpad.net/kicad/+bug/1805479
Locks protect the std::set in each item. Devolving the mutex to the
CN_ITEM allows multiple threads to make simultaneous connections to
different items where they do not conflict.
The connectivity files were unwieldy. This separates them logically
into data, algo and items where the items classes are those that hold,
surprise, surprise, the items, lists and clusters.
Seth Hillbrand