util/disk_cache: use LRU eviction rather than random eviction

util/disk_cache: use LRU eviction rather than random eviction

Still using fast random selection of two-character subdirectory in
which to check cache files rather than scanning entire cache.

v2: Factor out double strlen call
v3: C99 declaration of variables where used

Reviewed-by: Grazvydas Ignotas <notasas@gmail.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>