This syncs the checkpoint interface specification with the base class
(`BaseCheckpointSaver`) in
`langgraph/libs/checkpoint/langgraph/checkpoint/base /__init__.py`.
- Instead store sends in a Topic channel, removing the need to fetch sends as writes against the parent checkpoint
- Remove deprecated/unused functions in langgraph-checkpoint (will require bumping min range for langgraph-checkpoint in langgraph lib)
- Implement migration of old pending sends in langgraph-checkpoint-postgres
- Ensure parent config of `checkpoint_during=False` checkpoints always points to checkpoints that were also saved
