Re: [Proposal] Adding callback support for custom statistics kinds

Thanks for the updates!

> - Less fwrite() and fread(), more read_chunk() and write_chunk(). We
> are exposing these APIs, let's use them.

oops. That totally slipped my mind :( sorry about that.

> - The callbacks are renamed, to be more generic: "finish" for the
> end-of-operation actions and to/from_serialized_data.

At first I wasn’t a fan of the name “finish” for the callback.
I was thinking of calling it “finish_auxiliary”. But, we’re not
forcing callbacks to be used together, and there could perhaps
be cases where “finish" can be used on its own, so this is fine by me.

I made some changes as well, in v8:

1/ looks like b4cbc106a6ce snuck into v7. I fixed that.

2/ After looking this over, I realized that “extra” and “auxiliary”
were being used interchangeably. To avoid confusion, I replaced all
instances of “extra” with “auxiliary" in both the comments and
macros, i.e. TEST_CUSTOM_AUX_DATA_DESC

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Sami Imseih
Amazon Web Services (AWS)