post() with const ref.

Added "safety" of passing move-able reference
is an illusion since no use_count()==1 test
is done.  Instead extra (shallow) copies
were made for each subscriber.

Instead.  Pass const reference, redefine
MonitorControlOp::post() to transfer ownership,
and make only a single copy in SharedPV::post().

example/ticker.cpp

@@ -100,7 +100,7 @@ int main(int argc, char* argv[])
 
         val["value"] = count++;
 
-        pv.post(std::move(val));
+        pv.post(val);
 
         std::cout<<"Count "<<count<<"\n";
     }