Resize the connection cache upwards when adding more handles than what

currently fits in the cache, to make the cache work better especially for
pipelining cases but also for "mere" (persistent) connection re-use.

lib/multi.c

@@ -423,6 +423,18 @@ CURLMcode curl_multi_add_handle(CURLM *multi_handle,
 
   /* increase the node-counter */
   multi->num_easy++;
+
+  if((multi->num_easy+5) > multi->connc->num) {
+    /* we want the connection cache to have room for all easy transfers, and
+       some more so we have a margin of 5 for now, but we add the new amount
+       plus 10 to not have to do it for every new handle added */
+    CURLcode res = Curl_ch_connc(easy_handle, multi->connc,
+                                 multi->num_easy + 10);
+    if(res)
+      /* TODO: we need to do some cleaning up here! */
+      return res;
+  }
+
   /* increase the alive-counter */
   multi->num_alive++;