Workaround for cpu hog on errors with cached connections.

Just doing another connect isn't safe as peer.get() may expect peer.tries
to be strictly positive (this is the case e.g. with round robin with multiple
upstream servers).  Increment peer.tries to at least avoid cpu hog in
round robin balancer (with the patch alert will be seen instead).

This is not enough to fully address the problem though, hence TODO.  We
should be able to inform balancer that the error wasn't considered fatal
and it may make sense to retry the same peer.

src/http/ngx_http_upstream.c

@@ -2812,6 +2812,10 @@ ngx_http_upstream_next(ngx_http_request_t *r, ngx_http_upstream_t *u,
     if (u->peer.cached && ft_type == NGX_HTTP_UPSTREAM_FT_ERROR) {
         status = 0;
 
+        /* TODO: inform balancer instead */
+
+        u->peer.tries++;
+
     } else {
         switch(ft_type) {