Merge of r3966, r3967, r3968, r3969, r3970, r3971: Cache loader improvements: The cache loader performs two tasks: inserting cache objects in inactivity list and evaluating total cache size. Reading just directory is enough for this purpose. Elimination of reading cache files saves at least one disk I/O operation per file.

--- a/src/core/ngx_file.h
+++ b/src/core/ngx_file.h
@@ -104,6 +104,7 @@ typedef ngx_int_t (*ngx_tree_handler_pt)
 
 struct ngx_tree_ctx_s {
     off_t                      size;
+    off_t                      fs_size;
     ngx_uint_t                 access;
     time_t                     mtime;