fsck move should be non-destructive by default

The fsck move behavior in the code and originally articulated in HADOOP-101 is:

Current failure modes for DFS involve blocks that are completely missing. The only way to "fix" them would be to recover chains of blocks and put them into lost+found

A directory is created with the file name, the blocks that are accessible are created as individual files in this directory, then the original file is removed.

I suspect the rationale for this behavior was that you can't use files that are missing locations, and copying the block as files at least makes part of the files accessible. However this behavior can also result in permanent dataloss. Eg:

• Some datanodes don't come up (eg due to a HW issues) and checkin on cluster startup, files with blocks where all replicas are on these set of datanodes are marked corrupt
• Admin does fsck move, which deletes the "corrupt" files, saves whatever blocks were available
• The HW issues with datanodes are resolved, they are started and join the cluster. The NN tells them to delete their blocks for the corrupt files since the file was deleted.

I think we should:

• Make fsck move non-destructive by default (eg just does a move into lost+found)
• Make the destructive behavior optional (eg "--destructive" so admins think about what they're doing)
• Provide better sanity checks and warnings, eg if you're running fsck and not all the slaves have checked in (if using dfs.hosts) then fsck should print a warning indicating this that an admin should have to override if they want to do something destructive