ageLimit is not an ageLimit

If I'm not mistaken, when I set an ageLimit, TMCache trims cache by age and repeat the operation every "ageLimit" using dispatch_after.

If I set an ageLimit of 10 sec on t=0, insert an object on t=5, that object keeps in cache for 15 sec.

I think this is not the expected behavior.

Problem becomes worse when ageLimit becomes larger.
