Runtime filesystem consistency checking

Recon can catch bit flips that occur in memory and that corrupt file system metadata buffers before the Recon code is run. Catching bit flips after the Recon checks require checksumming and replication of file system metadata buffers. Recon will also not detect lost and misdirected writes if they occur after the Recon checks. However, they can be caught using version numbers, but the file system needs to support that, e.g., generation numbers in btrfs.