Add active thread logging

Closes #933

   * Add utility class `ThreadLogger` that can format and log the currently active Threads.
   * Used `ThreadLogger` in `Launcher` and `DefaultClientCloseHandler`

   Note that `ThreadLogger` is in a package in the `org.apache` namespace. I suggest we start using that namespace for new classes as we will have to move the whole codebase there eventually anyhow.