use associated executors for intermediate handlers

Summary: * per-operation cancellation changed: total/partial signals only prevent further resending, terminal signal cancels the whole client

Reviewers: ivica

Reviewed By: ivica

Subscribers: korina

Differential Revision: https://repo.mireo.local/D27246

test/unit/include/test_common/test_service.hpp

@@ -39,8 +39,7 @@ public:
 		CompletionToken&& token
 	) {
 		auto initiation = [this](auto handler) {
-			auto ex = asio::get_associated_executor(handler, _ex);
-			asio::post(ex,
+			asio::post(_ex,
 				asio::prepend(std::move(handler), error_code {})
 			);
 		};