Do not cleanup the cache during request handling

Previously, Mutalyzer would after writing any file check the cache size
and start removing files while it exceeded the maximum. However, this
caused long delays in case many files had to be removed (it would re-
calculate the total size after each removal).

Following the principle of separating concerns, this is now handled by
a separate script on our production servers, which uses the inotifywait
tool to cleanup the cache whenever files are added to it. It also
doesn't suffer from the performance problem.

Note that this removes the `MAX_CACHE_SIZE` configuration setting.

Fixes #18