Refactor ConfigurationClassEnhancer to allow cglib caching of
generated classes. Prior to this commit each enhanced @Configuration
class would consume permgen space when created.
The CallbackFilter and Callback Types are now defined as static final
members so that they can be shared by all enhancers. Only the
callbackInstances remain specific to a @Configuration class and
these are not used by cglib as part of the cache key.
Issue: SPR-9851
Backport-Commit: c8061393fb
ced6bb4569