util/disk_cache: actually enforce cache size

util/disk_cache: actually enforce cache size

Currently only a one in one out eviction so if at max_size and
cache files were to constantly increase in size then so would the
cache. Restrict to limit of 8 evictions per new cache entry.

V2: (Timothy Arceri) fix make check tests

Reviewed-by: Grazvydas Ignotas <notasas@gmail.com>