Performance improvement in generating the status window.

* The status window is refreshed every time a new thread is created.
  Every time a thread is created, the stacktrace of all threads are
  being recomputed (concatenated into string). In a large setup like
  Hadoop where there are about 200 threads, this takes a long time,
  especially when your debugging VM is remote. As a workaround, the
  status window is refreshed only after the debug target created event
  is received. This happens after all the existing threads have been
  notified. We still need to support sending incremental status updates.
  That is the long term solution.

* To achieve this, the initial state is set to CONNECTING. In this
  state, any thread creation event won't cause status to be refreshed.