transitionToActive blocks if the SBN is doing checkpoint image transfer

The NN and SBN do this dance during checkpoint image transfer with nested HTTP GETs via HttpURLConnection. When an admin does a -transitionToActive during this transfer, part of that is interrupting an ongoing checkpoint so we can transition immediately.

However, the thread.interrupt() in StandbyCheckpointer#stop gets swallowed by connection.getResponseCode() in TransferFsImage#doGetUrl. None of the methods in HttpURLConnection throw InterruptedException, so we need to do something else (perhaps HttpClient [1]):

[1]: http://hc.apache.org/httpclient-3.x/