Use 'cache-files-ttl' for cache gc, fixes #2441

The configuration option 'cache-ttl' was used instead of 'cache-files-ttl' to determine
whether or not a cache gc should be performed.

* changed 'cache-ttl' to 'cache-files-ttl' to determine if a gc should be performed
* refactored FileDownloader to allow for easier testing
* added test to ensure that the gc is called with the proper config option
Merge pull request #2453 from mirkobeine/#2441-V2

Use 'cache-files-ttl' for cache gc, fixes #2441