greentest.py: in dbg mode, call setUp() and tearDown() properly when executing tests multiple times for refcount

better start up procedure: check that we started by connecting to an app; also use this check to make sure no other app is bound to this socket

Do not handle QUIT, let Ctrl-\ work. Do not print stacktrace if target cannot be connected to.

test__threadpool.py: use built-in sleep() instead of gevent.sleep() in the pool, because otherwise a new Hub is created and screws up reference count checks

ThreadPool: fix _set_size to stop _fork_watcher if it's not needed; also make sure it does not try to sleep when run inside hub

This is to counter cases where SIGCHLD is not delivered or lost which I've seen happening with test_patched_signal.py