Add topology_identifier method to busgraph_cache

[boothby]

To support external logistics of embedding caches, we need a way of identifying a graph in a consistent manner. We already have functionality for that on a single host, but it's a largeish binary blob. For external use, we use the hexdigest of a sha256 of that binary blob.

Also contains a drive-by bugfix of #222

[boothby]

@dwave-bnugmanov that failed test is not a result of this patch, it's known to be a little noisy