Core: slab log_nomem flag. The flag allows to suppress "ngx_slab_alloc() failed: no memory" messages from a slab allocator, e.g., if an LRU expiration is used by a consumer and allocation failures aren't fatal. The flag is now used in the SSL session cache code, and in the limit_req module.

--- a/src/core/ngx_slab.h
+++ b/src/core/ngx_slab.h
@@ -39,6 +39,8 @@ typedef struct {
     u_char           *log_ctx;
     u_char            zero;
 
+    unsigned          log_nomem:1;
+
     void             *data;
     void             *addr;
 } ngx_slab_pool_t;