Keep a copy of R in job-manager for use in jobtap plugin callbacks

[grondo]

As noted in #3851, there are already a couple use cases where having the allocated R available to jobtap plugins would be convenient. In addition to those documented use cases, having R available from the job-manager will also be necessary in order to update the duration of a running job (#4175), since the current expiration will need to be available to create the new expiration necessary for the resource-update event.

This issue proposes that the job-manager keep a copy of R for jobs as they enter the RUN state. This may mean reworking the scheduler protocol to reply to an alloc request with R instead of putting R in the KVS and replying only after the commit. The job-manager could become the entity that puts R in the KVS or this can be done in parallel.

[garlick]

Have you already begun working on this? I'd be happy to prototype it and propose any RFC updates needed to RFC 27.

[grondo]

No, I haven't. Thanks @garlick!

[grondo]

This was fixed by #5472