thread: add way to notify waiting thread of changes

Don't notify on stop.

Change-Id: Ic3eead63c4756d1790da1ba7223a431cbc238232

src/threadloop.cpp

@@ -142,7 +142,6 @@ void
 InterruptedThreadLoop::stop()
 {
     ThreadLoop::stop();
-    std::lock_guard<std::mutex> lk(mutex_);
     cv_.notify_one();
 }
 } // namespace ring

src/threadloop.h

@@ -86,6 +86,11 @@ public:
 
     void stop() override;
 
+    void interrupt() noexcept
+    {
+        cv_.notify_one();
+    }
+
     template <typename Rep, typename Period>
     void
     wait_for(const std::chrono::duration<Rep, Period>& rel_time)