Initial implementation of fuse staging directory for Workspace Virt.

[Abrar Quazi (AbrarQuazi)]

Initial implementation for the staging directory in fuse daemon to get workspace virtualization working

• Implemented StagedFile struct with appropriate metadata for a representation, as well as relevaant mappings to a StagedFile
• Hooked up wakefuse file I/O operations to handle staged_files and inserts into the staged file directory
• Have Fuse output appropriate JSON:

• "ibytes": <integer>,
  "obytes": <integer>,
  "inputs": [
    "<path1>",
    "<path2>",
    ...
  ],
  "outputs": [
    "<path1>",
    "<path2>",
    ...
  ],
  "staging_files": {
    "<dest_path>": {
      "staging_path": "<path_in_staging_dir>",
      "mode": <integer>,
      "mtime_sec": <integer>,
      "mtime_nsec": <integer>
    },
    ...
  }
  

}```

• Have fuse client parse out the staging_files json, and reflink files to build/ directory