Fixes for guard/monitor in multi-cluster setup

[bob-r]

modified file: cluster.py

scheduler function (guard process): fix for #231, alternative solution to 87b87ff for MySQL

save_task function (monitor process): fix for #225

[codecov-io]

Codecov Report

Merging #248 into master will increase coverage by 0.3%.
The diff coverage is 72.72%.

@@            Coverage Diff            @@
##           master     #248     +/-   ##
=========================================
+ Coverage   89.88%   90.18%   +0.3%     
=========================================
  Files          44       42      -2     
  Lines        2866     2762    -104     
=========================================
- Hits         2576     2491     -85     
+ Misses        290      271     -19

Impacted Files	Coverage Δ
django_q/cluster.py	90.44% <72.72%> (-0.77%)	⬇️
django_q/conf.py	70.9% <0%> (-0.52%)	⬇️
django_q/tests/test_scheduler.py	100% <0%> (ø)	⬆️
django_q/tests/settings.py	100% <0%> (ø)	⬆️
django_q/brokers/aws_sqs.py	0% <0%> (ø)	⬆️
django_q/tests/test_cluster.py	100% <0%> (ø)	⬆️
django_q/tests/test_cached.py	100% <0%> (ø)	⬆️
django_q/core_signing.py
django_q/queues.py
... and 4 more

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[aleehedl]

As the SAVE_LIMIT doesn't seem to be respected in a multi-cluster environment, I would like to see this one merged. The commits in this PR obviously need some cleanup (probably 2 commits is enough).