transfer: improve Windows SO_SNDBUF update limit

- Change the 1 second SO_SNDBUF update limit from per transfer to per
  connection.

Prior to this change many transfers over the same connection could cause
many SO_SNDBUF updates made to that connection per second, which was
unnecessary.

Closes https://github.com/curl/curl/pull/12911

lib/transfer.c

@@ -781,9 +781,9 @@ static CURLcode readwrite_upload(struct Curl_easy *data,
 #if defined(_WIN32) && defined(USE_WINSOCK)
     {
       struct curltime n = Curl_now();
-      if(Curl_timediff(n, k->last_sndbuf_update) > 1000) {
+      if(Curl_timediff(n, conn->last_sndbuf_update) > 1000) {
         win_update_buffer_size(conn->writesockfd);
-        k->last_sndbuf_update = n;
+        conn->last_sndbuf_update = n;
       }
     }
 #endif

lib/urldata.h

@@ -721,10 +721,6 @@ struct SingleRequest {
   } p;
 #ifndef CURL_DISABLE_DOH
   struct dohdata *doh; /* DoH specific data for this request */
-#endif
-#if defined(_WIN32) && defined(USE_WINSOCK)
-  struct curltime last_sndbuf_update;  /* last time readwrite_upload called
-                                          win_update_buffer_size */
 #endif
   char fread_eof[2]; /* the body read callback (index 0) returned EOF or
                         the trailer read callback (index 1) returned EOF */
@@ -998,6 +994,11 @@ struct connectdata {
   CtxtHandle *sslContext;
 #endif
 
+#if defined(_WIN32) && defined(USE_WINSOCK)
+  struct curltime last_sndbuf_update;  /* last time readwrite_upload called
+                                          win_update_buffer_size */
+#endif
+
 #ifdef USE_GSASL
   struct gsasldata gsasl;
 #endif