Allow BlockingVariable to be reused by adding a reset() method

Originally reported on Google Code with ID 190

I have found it very useful to be able to reuse a blocking variable once it has been
used once.

I accomplished this with two minor compatible changes to the current BlockingVariable.java

change....

     private CountDownLatch valueReady = new CountDownLatch(1);

to be not final

add the method

    public void reset() {
          value= null;
          valueReady= new CountDownLatch(1);
    }

Then a call to reset() will allow the same blocking variable to be reused.

Reported by wolfmanjm on 2011-07-19 06:58:59

Start by locating and reading BlockingVariable.java, especially the value and CountDownLatch fields described in the issue. Add the requested reset behavior so the same blocking variable can be used again after completion, then verify that the existing behavior remains compatible and that a reset allows another value to be received.