flux account-priority-update: add max-preemptible-after to bank information sent over to plugin

[cmoussa1]

Once #553 is completed, the bank information that is sent to the priority-plugin via flux account-priority-update should also include the max-preemptible-after information for a bank.

The plugin should be responsible for unpacking this attribute and storing it. Because the plugin does not store general bank information right now, perhaps I could look into adding a new Bank object or something that is composed of just the name of the bank and its max-preemptible-after value (if any). Then, the plugin can keep an internal map of Bank objects and reference this when looking to either set a preemptible-after default or enforce a preemptible-after limit on an association's job.