plugin: support bypassing limits

[cmoussa1]

This was brought up in our team meeting yesterday that eventually, the plugin should be able to support bypassing limits for certain users' jobs. This could be handled (at least at first) by unpacking and checking for a certain attribute in the callbacks that increase/decrease limits.

For example, in job.new, if we find this attribute, we can skip this increment of the users' active_jobs count:

flux-accounting/src/plugins/mf_priority.cpp

Line 827 in b291ffb

b->cur_active_jobs++;

Or ignore a user's running_jobs count by skipping the increment in the callback for job.state.run:

flux-accounting/src/plugins/mf_priority.cpp

Line 908 in b291ffb

b->cur_run_jobs++;