add Context::close()

src/clientdiscover.cpp

@@ -82,6 +82,9 @@ std::shared_ptr<Operation> DiscoverBuilder::exec()
 
     context->tcp_loop.dispatch([op, context, ping]() {
 
+        if(context->state!=ContextImpl::Running)
+            throw std::logic_error("Context close()d");
+
         bool first = context->discoverers.empty();
 
         context->discoverers[op.get()] = op;