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/core/ngx_palloc.h
+++ b/src/core/ngx_palloc.h
@@ -46,6 +46,7 @@ typedef struct {
     u_char               *last;
     u_char               *end;
     ngx_pool_t           *next;
+    ngx_uint_t            failed;
 } ngx_pool_data_t;