DistCache: executable flag, dir support

[zentol]

new features:

• you can now distribute whole directories (this copies all files tracked by listStatus recursively)
• when distributing single files they can be tagged as executable (automatically checked for local files)

other changes:

• the name/path scheme was changed to preserve the original name. all files are now stored in a job-specific folder, instead of having a job/identifier specific name.
• the arguments passed when registering a file are abstracted away in its own data structure, making it easier to expand the DC the future. (there is no reason why one should have to change NepheleJobGraphGenerator or TaskManager for this)
• reworded/expanded some javadocs, and made it clear that local files can only be used when they are accessible form all workers

[rmetzger]

here, you could have also done if(content.isDir() && distPath.endsWith("/")) to save one if statement.
But you don't need to update the PR for that ;)

[StephanEwen]

This is merged in f952638