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/mailbox.cpp

@@ -74,7 +74,7 @@ int main(int argc, char* argv[])
 
         // (optional) update the SharedPV cache and send
         // a update to any subscribers
-        pv.post(std::move(top));
+        pv.post(top);
 
         // Required.  Inform client that PUT operation is complete.
         op->reply();