Make Joblet a Callable/Runnable

[roshan]

If we make a Joblet a Callable and add a wrapper to turn a Callable into a Joblet, then we can take code that was previously being submitted to an ExecutorService and just turn it trivially into a Joblet.

This will require a change of the interface to use a call method. Alternatively, if we want a more gradual change, we could just as well make it a Runnable and force thrown exceptions to be RuntimeExceptions. However, if we want to support catching a more diverse variety of errors we could just as well make it identical to Callable#call .

[roshan]

@armaansarkar an alternative is to have code branch on instanceof Runnable and instanceof Callable. What do you think?