Previously onStop() was called before joining the acceptor thread, so teardown could destroy resources while the thread running onStart() was still using them. Move onStop() after thread_join() so the thread is fully joined before teardown.

onStop() is the derived-class hook that tears down acceptor resources (e.g.
SocketAcceptor closes its listening sockets and destroys the socket server).
The acceptor's background thread runs onStart() and keeps using those
resources until it actually exits.

In the previous order, stop() set m_stop = true and then immediately called
onStop() while the worker thread was still running. This created a race
between teardown and the still-live onStart() loop:

• onStop() could destroy sockets / server state that the worker thread was
  concurrently reading from or writing to, leading to use-after-free and
  undefined behavior on shutdown.
• The subsequent thread_join() then waited on a thread that had been racing
  against destruction of the very objects it depended on.

Setting the m_stop flag, joining the thread, and only then running onStop()
guarantees the worker thread has fully exited before any of its resources are
released — eliminating the race.