The cache loader performs two tasks: inserting cache objects in inactivity

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.

Preparation for elimination of reading cache files by cache loader:
removing dependencies on the reading:
*) cache node valid_sec and valid_msec are used only for caching errors;
*) upstream buffer size can be used instead of cache node body_start.