2 - Switch from NEXT to MRO::Compat.
3 - Change config key from 'cache' to 'Plugin::Cache', old key
4 is still supported for backwards compatibility, but the new
5 key is preferred (RT#40344).
8 - clarify the documentation on how to configure a backend
9 - due to stacked evals, certain start up errors were quietly
10 ignored until the first cache hit. These start up errors
11 now correctly manifest themselves at start up time.
13 - change base class order to appease Class::C3
19 - Be more compatible with ->set() convention of passing expiry
22 - Pass around a hash ref instead of a list of kvp.