Snapshot checksum verification

Adds automatic verification of all files that are being snapshotted. Closes #5593
Yes, that test takes care of my concerns. Maybe we can just add a comment to readBytes that our casts to 'int' are safe because seek catches us up? I stared hard at this thing, I dont see any problems, but it might help someone else. Thanks for doing this, I think it may be useful in the future for other purposes too (e.g. fold logic into ChecksumIndexInput).