osd: switching dispatch_running to be a counter

Now it works with multiple messengers dispatching.

src/osd/OSD.cc

@@ -1846,9 +1846,9 @@ bool OSD::ms_dispatch(Message *m)
 {
   // lock!
   osd_lock.Lock();
-  dispatch_running = true;
+  ++dispatch_running;
   _dispatch(m);
-  dispatch_running = false;
+  --dispatch_running;
   do_waiters();
   osd_lock.Unlock();
   return true;

src/osd/OSD.h

@@ -130,7 +130,7 @@ protected:
     }
   };
 
-  bool dispatch_running;
+  int dispatch_running;
 
   void open_logger();
   void start_logger();