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/spam.cpp

@@ -62,7 +62,7 @@ public:
 
                     log_debug_printf(app, "%s count %u\n", sub->peerName().c_str(), unsigned(cnt));
 
-                }while(sub->tryPost(std::move(update)));
+                }while(sub->tryPost(update));
             };
 
             sub->onHighMark(fill);