fix(checkpoint-postgres): store channel_values correctly

[benjamincburns]

Prior to this change, PostgresSaver wouldn't store channel_values if newVersions wasn't passed to the put method. This is due to a filter check in _dumpBlobs that checks each channel name to ensure it appears in the channel versions object prior to serializing it for storage.

Upon inspecting the other CheckpointSaver types, I noticed that MemorySaver, MongoDBSaver, and SqliteSaver) all ignore the newVersions argument to the BaseCheckpointSaver.put method, so I think this is the correct fix. From what I can see by searching references, no test code (apart from the tests this PR changes) passes the newVersions argument.

That said, the one reference to BaseCheckpointSaver.put that I found in actual shipped code is in PregelLoop._putCheckpoint (via _checkpointerPutAfterPrevious), and this does pass the newVersions argument.

So I have to ask, are all of the other CheckpointSaver implementations, and all of the existing tests implemented incorrectly, or is this the correct fix?

Found this while working on #541, thanks to the getTuple tests (all of them failed when I wired them up to PostgresSaver).

[hgoona]

@benjamincburns are you saying the current PostgresSaver implementation is faulty? (Trying to understand which checkpoint Saver to use as a reference to develop my own one. It's so very confusing. I'm still trying to fix mine and hitting errors.. )

[benjamincburns]

I don't know whether it is faulty or not. It looks to me like either it is faulty, or the other three are. It seems unlikely that it would be the other three, especially since this one was just introduced, but I can't say for certain, which is why I asked.

[jacoblee93]

Hey @benjamincburns, respecting newVersions is the correct thing to do. Thanks so much for flagging this!

Let's close this one, if you can open an issue for making other implementations respect this that would be helpful, otherwise I can.

[benjamincburns]

@jacoblee93 ah, glad I asked! I'll take a crack at it and reply back here if I'm unable to make it work for some reason.