StateStore store() {
            return this.stateStore;
        }
ProcessorStateManager is the source of truth for the current offset for each state store,
which is either the read offset during restoring, or the written offset during normal processing.

The offset is initialized as null when the state store is registered, and then it can be updated by
loading checkpoint file, restore state stores, or passing from the record collector's written offsets.

When checkpointing, if the offset is not null it would be written to the file.

The manager is also responsible for restoring state stores via their registered restore callback,
which is used for both updating standby tasks as well as restoring active tasks.
streams/src/main/java/org/apache/kafka/streams/processor/internals/ProcessorStateManager.java