Support file size based usage limits

[lozbrown]

Currently rubix configuration only supports integer percentages for cache limits in the following configuration item

rubix.cache.usage.percentage

However when using an elastic storage like amazon EFS this presents as 8 Exabytes of available storage. Even limiting the cache usage to 1% gives us a 90 Terabyte cache which is extremely excessive.

Support limiting the cache to some number of bytes/megabytes/gigabytes and evict data when either limit is reached.