Exploit fast "multiple paths from same source" in distribution

Exploit fast "multiple paths from same source" in distribution

Dijkstra's algorithm can find multiple paths from the same source.
This is much faster than starting from scratch for every path.

Make distribution path assembly work that way.  This speeds up runs of
distributions to the same center.  The next commit will reorder path
searches to maximize the length of these runs.  It also has benchmark
results.

Allocates four bytes per sector, actually uses only the first 4*n
bytes, where n is the number of distributing sectors.