Benchmark the Snapshot code
We've now got code to take snapshots of siafiles, but there are some pretty deeply nested loops in that code, and I think it would be good to have some benchmarks around the code to make sure it's safe to be using those methods freely. I believe that it is, and that a snapshot should still take well under a millisecond to grab even for large siafiles, but we should verify this before assuming it and programming as though it is the case.