ALLJOYN-1964 AllJoynPerfTest app on iOS exits abruptly after completion of
data transfer using signal or method calls

1. Any call to a update an UI element in iOS must happen on the main thread.
Inside didTouchStartButton there was a call to update the eventTextView
which was not on the main thread which caused it to crashed.
2. Updated the call to disconnect from the bus inside the AJNServiceController
3. Updated the behavior of Start/Stop button for the receiver so that the
performance statistics are not wiped off after completion of data transfer