Introduce a flag to switch to a new serialization strategy

With we simplified the fields needed to encode IdSourceKey, which certain dialects serialize directly into a buffer stored by the data store (e.g. Ehcache and Infinispan embedded).

We decided to not break serialization compatibility, however we should introduce a flag or a strategy to eventually move on to the leaner format.

Especially new applications should be encouraged to use the new schema.

See Ehcache's SerializableIdSourceKey and Infinispan's IdSourceKeyExternalizer