More performance improvements in checkpointing and channels (#1685)

* Performance improvements in checkpointer libs

- Use sha1 instead of md5 for hashing (faster in python 3.x)
- Use orjson instead of json for json dumping (sadly can't use for json loading)

* Update tests

* Update

* Use random number instead of hash for get_version_number

* Avoid saving writes for the last task to complete in each step

- only when possible, exceptions for ERROR, INTERRUPT, SEND

* Make Channel.from_checkpoint a regular function

- context manager no longer needed since Context became a managed value

* Use __slots__ for Channels

* Fix for kafka

libs/checkpoint-postgres/langgraph/checkpoint/postgres/base.py

@@ -1,4 +1,4 @@
-from hashlib import md5
+import random
 from typing import Any, List, Optional, Tuple
 
 from langchain_core.runnables import RunnableConfig
@@ -8,7 +8,6 @@ from langgraph.checkpoint.base import (
     WRITES_IDX_MAP,
     BaseCheckpointSaver,
     Checkpoint,
-    EmptyChannelError,
     get_checkpoint_id,
 )
 from langgraph.checkpoint.serde.jsonplus import JsonPlusSerializer
@@ -244,11 +243,8 @@ class BasePostgresSaver(BaseCheckpointSaver):
         else:
             current_v = int(current.split(".")[0])
         next_v = current_v + 1
-        try:
-            next_h = md5(self.serde.dumps_typed(channel.checkpoint())[1]).hexdigest()
-        except EmptyChannelError:
-            next_h = ""
-        return f"{next_v:032}.{next_h}"
+        next_h = random.random()
+        return f"{next_v:032}.{next_h:016}"
 
     def _search_where(
         self,