Remove minimum file cache size restriction

[andrross]

The current file cache factory has the requirement that each cache segment must be larger than the block size. This restriction was put into place because the initial behavior of the cache would immediately close a newly added entry if the cache was full. This meant that if the segment size was smaller than the block size then nothing would work. In practice this was never really an issue because useful cache sizes are multiple orders of magnitude larger than the block size. However, now that #6592 has been implemented this restriction can be removed. The restriction isn't harmful per se, but it can make testing awkward, so it should be removed. The code in question is here. This task is to remove that condition and update tests accordingly.