Remove Checkpoint.pending_sends

- 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

libs/checkpoint-sqlite/README.md

@@ -12,7 +12,7 @@ read_config = {"configurable": {"thread_id": "1"}}
 
 with SqliteSaver.from_conn_string(":memory:") as checkpointer:
     checkpoint = {
-        "v": 2,
+        "v": 4,
         "ts": "2024-07-31T20:14:19.804150+00:00",
         "id": "1ef4f797-8335-6428-8001-8a1503f9b875",
         "channel_values": {
@@ -34,7 +34,6 @@ with SqliteSaver.from_conn_string(":memory:") as checkpointer:
                 "start:node": 2
             }
         },
-        "pending_sends": [],
     }
 
     # store checkpoint
@@ -54,7 +53,7 @@ from langgraph.checkpoint.sqlite.aio import AsyncSqliteSaver
 
 async with AsyncSqliteSaver.from_conn_string(":memory:") as checkpointer:
     checkpoint = {
-        "v": 2,
+        "v": 4,
         "ts": "2024-07-31T20:14:19.804150+00:00",
         "id": "1ef4f797-8335-6428-8001-8a1503f9b875",
         "channel_values": {
@@ -76,7 +75,6 @@ async with AsyncSqliteSaver.from_conn_string(":memory:") as checkpointer:
                 "start:node": 2
             }
         },
-        "pending_sends": [],
     }
 
     # store checkpoint