Check the cache every <time> hours, and delete files if
the space usage is greater than that set by CacheSize. Note that
<time> accepts a float value, you could for example use
CacheGcInterval 1.5 to check the cache every 90
minutes. (If unset, no garbage collection will be performed, and
the cache will grow indefinitely.) Note also that the larger the
CacheGcInterval, the more extra space beyond the
configured CacheSize will be needed for the cache
between garbage collections.