http: bugfix: track closed connection

It is possible that the client disconnects before the request is
handled. In those cases, evhttp_request_set_on_complete_cb is never
called, which means that on shutdown the server we'll keep waiting
endlessly.

By adding evhttp_connection_set_closecb, libevent automatically
cleans up those dead connections at latest when we shutdown, and
depending on the libevent version already at the moment of remote
client disconnect. In both cases, the bug is fixed.

src/httpserver.cpp

@@ -262,19 +262,22 @@ std::string RequestMethodString(HTTPRequest::RequestMethod m)
 /** HTTP request callback */
 static void http_request_cb(struct evhttp_request* req, void* arg)
 {
+    evhttp_connection* conn{evhttp_request_get_connection(req)};
     // Track active requests
     {
         g_requests.AddRequest(req);
         evhttp_request_set_on_complete_cb(req, [](struct evhttp_request* req, void*) {
             g_requests.RemoveRequest(req);
         }, nullptr);
+        evhttp_connection_set_closecb(conn, [](evhttp_connection* conn, void* arg) {
+            g_requests.RemoveConnection(conn);
+        }, nullptr);
     }
 
     // Disable reading to work around a libevent bug, fixed in 2.1.9
     // See https://github.com/libevent/libevent/commit/5ff8eb26371c4dc56f384b2de35bea2d87814779
     // and https://github.com/bitcoin/bitcoin/pull/11593.
     if (event_get_version_number() >= 0x02010600 && event_get_version_number() < 0x02010900) {
-        evhttp_connection* conn = evhttp_request_get_connection(req);
         if (conn) {
             bufferevent* bev = evhttp_connection_get_bufferevent(conn);
             if (bev) {