improve performance of blockimgdiff

Substantially improves the performance of some BlockImageDiff steps on
large images.  GenerateDigraph and AssertSequenceGood are now linear in
the number of blocks in the target image rather than quadratic in the
size of the partition.  FindVertexSequence tracks the score of each
vertex, the set of sources and sinks, and maintains a heap for finding
the best vertex to use for the greedy step, instead of repeatedly
re-scanning the set of remaining vertexes.

Change-Id: Ifeea5053d80ef4f06a5aa239c9a1797dd5e47841