implement time to live

[pavarnos]

see #24

• many tests broken because they test implementation details (the contents of the file on disk). Would it make sense to just test behaviour (the CacheInterface)?
• i copied Filesystem::now() from ArrayCache::now(). If now() was protected, maybe we could make a FakeFilesystem to override now() with a value we control for testing purposes... except that the Filesystem class is final...
• this change makes existing caches on disk incompatible: so if you release it needs to be a new major version?
• maybe need a clean() method which would look for all files on disk, read them all, then delete the expired ones

[WyriHaximus]

• many tests broken because they test implementation details (the contents of the file on disk). Would it make sense to just test behaviour (the CacheInterface)?

Yes, it took me longer to learn this then I'd like to admit but, only test for behaviour and mock as little as possible.

[WyriHaximus]

Will do a follow up fixing any outstanding issues for this PR, thanks for taking the time to put this together 👍

[WyriHaximus]

@pavarnos FYI: #29