merge r2889, r2918, and r2921: fix a pool growing and CPU usage for multi-requests keepalive connections: *) keepalive_requests *) try to reuse last 4 free large allocation links *) do not test a pool block space if we can not allocated from the block 4 times

--- a/src/http/ngx_http_request.c
+++ b/src/http/ngx_http_request.c
@@ -259,6 +259,8 @@ ngx_http_init_request(ngx_event_t *rev)
         return;
     }
 
+    c->requests++;
+
     hc = c->data;
 
     if (hc == NULL) {