Guard last_event_id() against the async event loop in SSEClient (#2644)

last_event_id() returned a reference to a string that run_event_loop() assigns on its own thread under start_async(), so a caller reading it from another thread raced the writes. The member is now guarded by a mutex and the getter returns a copy.

The return type changes from const std::string & to std::string. Code that binds the result to a const reference or copies it still compiles.

README-sse.md

@@ -100,7 +100,7 @@ sse.stop();
 
 ```cpp
 bool connected = sse.is_connected();
-const std::string &id = sse.last_event_id();
+std::string id = sse.last_event_id();
 ```
 
 ## Examples