Store SSTable checksums

[manishrjain]

We should checksum SSTables as they get built, and store them as the last 4/8 bytes in the tables, or as an additional field in the manifest.

That way, if the tables got their bits flipped, we'd know. We can make the badger info tool do the checksum matches.

#671 seems like it could be due to a data change in .sst file.