DAP Integration: Batched submissions

[martinthomson]

The DAP protocol is presently structured so that the leader accepts submissions directly (and optionally, via Oblivious HTTP).

Our design has the advertiser site, or their delegate, collect a bunch of submissions before submitting them in batches. Batching submissions makes it less necessary to use something like Oblivious HTTP. It also offers a number of optimization opportunities. Most -- if not all -- submissions will share metadata, so it would make sense to ensure that metadata is only transmitted once. That is not currently compatible with the structure of submissions defined in DAP, but a batch submission extension should be possible.

[cjpatton]

We considered this feature for DAP. In the early days, @ekr and others objected, noting that this could be handled at the HTTP layer. (The HTTTP client could send many upload requests simultaneously on the same HTTP stream.)

That said, I'd be supportive of this change if it makes your life easier. We could either:

1. Add it to the base draft (please file an issue at https://github.com/ietf-wg-ppm/draft-ietf-ppm-dap/issues).
2. Define this DAP endpoint in a new document.

I don't have a preference.