Fix logic for multiple job resize operations.

The original logic had a problem if you shrank a job and later grew it.
Nodes previously released would reappear when the job grows, but have
zero CPUs associated with them. The problem was due to the original nodes
list of a job being preserved in the job_resources data structure. The
new logic confirms that those nodes are still in the job's allocation
before rebuilding the job_resources data structure.