Support multiple subscription identifiers in received messages.

Summary:
- refactor property encoding
- change user property type to be std::pair<std::string, std::string>

Reviewers: ivica

Reviewed By: ivica

Subscribers: korina

Differential Revision: https://repo.mireo.local/D27867

include/async_mqtt5/impl/endpoints.hpp

@@ -141,8 +141,8 @@ class endpoints {
 
 public:
 	template <typename Executor>
-	endpoints(Executor ex, asio::steady_timer& timer)
-		: _resolver(ex), _connect_timer(timer)
+	endpoints(Executor ex, asio::steady_timer& timer) :
+		_resolver(ex), _connect_timer(timer)
 	{}
 
 	void clone_servers(const endpoints& other) {