Upload asset chunks concurrently

[ericswanson-dfinity]

This was not possible through the bootstrap, for two reasons:

• the bootstrap only allowed one connection (though this still allowed multiple requests to execute within a single block)
• idle connections leaving agent in a state where all requests failed with a "channel closed" error after a http/408 timeout response

It's uncertain whether this will be possible post-bootstrap, but somehow, someday, we need to be able to upload more than one chunk per block.

Splitting up calls to call_and_wait() into many call() before waiting for the requests to finish would bypass the idle connection issue.

[sesi200]

@smallstepman do you by chance already have this on your radar?

[ericswanson-dfinity]

This has been implemented: https://github.com/dfinity/sdk/blob/master/src/canisters/frontend/ic-asset/src/batch_upload/plumbing.rs#L359