util/disk_cache: use a helper to compute cache keys

util/disk_cache: use a helper to compute cache keys

This will allow to hash additional data into the cache keys or even
change the hashing algorithm easily, should we decide to do so.

v2: don't try to compute key (and crash) if cache is disabled

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