Gracefully dump stats before exiting on Ctrl-C

[jonathanzxu]

Users can only get cumulative recall and latency metrics at the very end of a run, after completion; this is sometimes wasteful as information is lost if the user kills the process early. Allow VSB to catch a SIGINT and dump useful cumulative stats before exiting.

[daverigby]

This probably boils down to a revert of 66d6d42, perhaps adding a warning message if the stats are partial (environment.shape_class.finished is False).