2881: API server associates nodes with their assigned jobs.

This will enable us to write better administrative tools (like the
Node Manager) and dashboards.
2881: crunch-dispatch updates nodes' job assignments.

With this commit, crunch-dispatch updates all the information that
update_node_attributes used to, so it's removed.
Merge branch '2881-node-has-job-wip'

Refs #2881.  Closes #3944.