[bugfix] Fix ReFrame hanging when Slurm job gets OOM killed

[vkarak]

Also:

• Remove the JobState abstraction, because it does not offer anything for the moment.
• Update Slurm job states.

Fixes #833.

[codecov-io]

Codecov Report

Merging #835 into master will decrease coverage by 0.03%.
The diff coverage is 39.13%.

@@            Coverage Diff             @@
##           master     #835      +/-   ##
==========================================
- Coverage   91.87%   91.84%   -0.04%     
==========================================
  Files          80       80              
  Lines       10277    10246      -31     
==========================================
- Hits         9442     9410      -32     
- Misses        835      836       +1

Impacted Files	Coverage Δ
reframe/core/schedulers/local.py	98.87% <100%> (-0.06%)	⬇️
reframe/core/schedulers/__init__.py	95.83% <100%> (+1.63%)	⬆️
reframe/core/schedulers/slurm.py	55.92% <13.33%> (-3.15%)	⬇️
unittests/test_schedulers.py	97.65% <75%> (-0.02%)	⬇️

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[teojgo]

lgtm