Allowing checkpointers' serializers to be asynchronous

[wpoynter]

Updated the SerializationProtocol interface to allow dumpsTyped and loadsTyped to be run synchronously or asynchronously.
Applied await to calls of each function throughout the existing checkpoint savers and tests.

This should allow future users to provide asynchronous serialization and deserialization for their savers, without breaking any current implementations.

[jacoblee93]

This will break folks using custom checkpointers - it's early days but I'd prefer not to. Will think through a backwards compatible way to do this.

[wpoynter]

This will break folks using custom checkpointers - it's early days but I'd prefer not to. Will think through a backwards compatible way to do this.

@jacoblee93 Not a problem, but can you explain how this will break existing users with custom checkpointers? I think I am missing something that it would be worth me learning.

[jacoblee93]

I mean this will be breaking if you've implemented your own custom checkpointer (which I know a few have) since they'll need to change their code to await serde methods.

[benjamincburns]

This will break folks using custom checkpointers - it's early days but I'd prefer not to. Will think through a backwards compatible way to do this.

I agree with and support your preference to avoid a breaking change here. That said, if a breaking change is necessary, I think it would be a bit cleaner to remove the serialization stuff from BaseCheckpointSaver altogether, as this seems like it's more fitting as an internal detail of the concrete implementation, rather than a requirement of all classes that extend BaseCheckpointSaver.