make all jobs "waitable"

[garlick]

Problem: one must submit a job with --flags=waitable in order to use flux job wait on it.

Originally, jobs were removed from the job manager upon transition to inactive but kept in the KVS. The waitable flag caused them to be kept in the job-manager until they they were reaped with flux job wait. Since that impacted the job manager memory footprint, it was neither the default, nor permitted for guests.

But now, the job manager keeps inactive jobs in memory and only removes them when they are purged from the system so the waitable flag has no impact on the job manager memory footprint. To improve the usability of flux job wait we might want to make all jobs waitable by default.

In addition, we should consider whether guests should be permitted to wait on the jobs they own.

[garlick]

This was implemented in #5748 but deemed not really worth the extra complexity.