job-list: stream responses

[chu11]

The job-list module returns job-list results in a giant blob, which can get quite large on big systems. This can cause HOL blocking.

Data could / should be streamed instead.

[garlick]

RFC 43 would need an update too.

[chu11]

hmmm, for backwards compatibility, should we stream only if the request is a streaming rpc?

I think low-probability someone is calling job-list directly ... but it is possible.