Slurm Collector: Handle job state / missing timestamps

[rkleinem]

Fist part of the problem

Currently the collector naively takes a list of job states from the config file and tries to collect all corresponding jobs.
But obviously not all job states make sense in this context. E.g. Pending makes no sense.
I think that we should document a list of job states that can be sensibly used with this collector.

Second part

There are job states that are a little more involved, like Cancelled. You might want to account for cancelled jobs when they were cancelled after running for a few days.
On the other hand there is no guarantee that a cancelled job was ever started. In this case the start_time is Unknown and tokenization of the sacct output will fail at

AUDITOR/collectors/slurm/src/sacctcaller.rs

Line 206 in 256395c

let v = match kc.key_type.parse(&v) {

I believe we need to define which fields might be missing for what job states and have the collector ignore certain entries. (Like a Cancelled job with no start_time instead of crashing.
I didn't think the list through (https://slurm.schedmd.com/sacct.html#SECTION_JOB-STATE-CODES). Cancelled might be the only problem.