* 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
* Fix semantics of put_writes/list
- put_writes(error) should not prevent saving future successful if task is retried successfully
- put_writes(writes) should be a no-op if non-error writes already exist for that task (this prevents tasks executed more than once from modifying writes previously saved / acted on)
- checkpoints should not include channel default values (ie those without a version)
- list() should fetch and return writes for each checkpoint
* Lint
* Rm print
* Fix import
* Lint
