Make Worker::run(mxb::Semaphore*) private

The semaphore is only used when Worker::start() is called to synchronize
the startup of the two threads. Also asserted that the state is what we
expected it to be.

server/core/worker.cc

@@ -529,6 +529,7 @@ bool Worker::post_message(uint32_t msg_id, intptr_t arg1, intptr_t arg2)
 
 void Worker::run(mxb::Semaphore* pSem)
 {
+    ss_dassert(m_state == STOPPED);
     this_thread.pCurrent_worker = this;
 
     if (pre_run())