TP: internal thread pool is not shut down

In TP.destroy(), the regular thread pool and timer are shut down, but not the internal thread pool. The result is that sometimes closing a channel takes a longer time (the max idle time for a thread in the pool). This only happens when an internal thread is active at the time of shut down.

Also use a separate thread factory for internal threads, so we can see by the naming whether a thread is regular or internal, e.g. "jgroups-23" (regular) versus "jgroups-int-33"