In writeFile, fix timeout bug by changing to spawn a separate thread to write the output.
Details: writeFile was trying to send the data on the same thread for servicing incoming interests and was getting blocked.
When it receives the interest, ccnfileproxy gets the file and writes out all the segments which are ultimately stored in the CCNFlowControl object.
When it tries close the output, this eventually calls CCNFlowControl.waitForPutDrain which expects all its output content to have been requested and serviced.
But because this wait loop is on the same interest service thread, no interests are serviced and the attempt to close times out after 10 seconds.
This returns control to the interest service thread and the content is sent.
The solution is to spawn a separate thread to write the output segments and not block the incoming interest service thread.