FlowExecutionList.Storage

[jglick]

Subsumes #368 & #369. Supersedes #312.

[jglick]

Apparently forgotten earlier. Even though this is abstract and has no fields, and the sole implementation is WorkflowRun.Owner which did specify an explicit serialVersionUID, Java serialization requires that all superclasses running from the concrete implementation class up to the topmost class implementing Serializable each have a stream class description including a compatible SVUID. Adding the getExternalizableId method changes the computed SVUID for the superclass, despite obviously making no changes to the actual serial form, so the previous effective value must be restored.

(Noticed via a proprietary test which involved serializing a WorkflowRun.Owner using a historical release of this plugin and then deserializing it in the current version. CC @gbhat618)