This proposes to refine the OperatorStateStore interface by,
- deprecating Java serialization shortcuts
- rename getOperatorState to getListState
The Java serialization shortcuts can be deprecated because they were
previously introduced to provide a smoother migration path from older
savepoints. However, its usage should definitely be discouraged.
Renaming to getListState is a preparation of making the names of state
access methods contain information about both its redistribution pattern
on restore and the shape of its data structure, since the combination of
these two is orthogonal. This convention will also provide a better
naming pattern for more state access methods in the future, for example
getUnionListState. If the method name does not contain its
redistribution pattern (e.g., getListState), then it simply implies the
default repartitioning scheme (SPLIT_DISTRIBUTE).