refactor lua job_submit bindings

use native tables for everything that you can just index without any
setup, so user scripts don't require any boilerplate setup code for them
add reservation state in slurm.reservations
reload lua job_script script when its mtime changes
lua memory leak fix

Close old lua file once a new one is loaded
Update job_submit lua examples
Explicitly define function args to be void
Describe job_submit/lua changes