v1.3.0: add MaxIdleAge(time.Duration) and Clear

v1.2.1: allow MaxErrAge(0) or MaxErrAge(-1) to expire immediately

Previously, 0 or -1 caused errors to be cached forever.
This was not the intent.

v1.2.0: retract v1.1.0

v1.1.0: add cache.SetCleanFn

v1.0.0: tag a stable API