Serialization of generic types in operators with state

[kyungissac]

Currently, there are operators that have generics in their states. If these are not Serializable, the serialization process will inevitably fail.

• ReduceByKeyOperator: Key/Value of HashMap
• ApplyStatefulOperator: state of the ApplyStatefulFunction (not exactly a generic, but likewise must implement Serializable)
• FixedSizeWindowOperator, CountWindowOperator, SessionWindowOperator : the type of data in Window

We must address these in a separate way.