add preemptible support to flux-accounting

[cmoussa1]

It was discussed in our production meeting today that perhaps flux-accounting should play a role in supporting preemptable jobs in a system instance of Flux. One idea discussed adding attributes to banks so that they only allow preemptable jobs.

Tagging @ryanday36 here to see what additional requirements might be needed that flux-accounting would be able to support.

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1. bank_table: add max-preemptible-after column #553
   4 of 4
   commands database new feature +3
   
2. bank_table: add max-preempt-after column #556
   database new feature +2
3. flux account-priority-update: add max-preemptible-after to bank information sent over to plugin #554
   commands new feature +2
   
4. plugin: enforce preemptible-after limits, set preemptible-after default #555
   new feature plugin +2
   

[ryanday36]

I think that what I'd like for this would be a 'max-preempt-after' value for banks. Values would be something like:

-1 (or null or whatever): no preemption required for the bank
0 or a flux standard duration time value: jobs using this bank get --preempt-after this value added unless they've already specified a smaller value of --preempt-after. E.g. if max-preempt-after is set to two hours, jobs that don't set --preempt-after or that set something like --preempt-after=4h will get --preempt-after=2h set, but jobs that set something like --preempt-after=1h would not be changed.

[cmoussa1]

Thanks for this explanation @ryanday36 - I'm going to convert this to a tracking issue and begin opening sub-issues with more specific details.