Runtime filesystem consistency checking

We are aware of the btrfs integrity checker and plan to incorporate some ideas from there into Recon. There are two main differences between the btrfs integrity checker and Recon right now. The btrfs integrity checker depends on the file system code while Recon works outside, at the block layer. A bug in the file system may affect the integrity checker, while the hope is that bugs in the file system and in Recon are going to be unrelated. Second, the integrity checker mainly checks that updated blocks are destined to the correct locations, while Recon checks the contents of the updated blocks.