Feature: save_limit per task group

[jonaswinkler]

This implements the idea presented in #491.

To summarize the issue:

• I've got tasks that execute every 10 minutes. (group1)
• I've got tasks that execute once every week. (group2)
• I'd like to keep success messages of both groups in the database, without configuring save_limit to something very high (10000+ would be required here). It's also somewhat annoying that making a sane configuration here depends on the actual number of groups and their schedule.

Impact:

• Nothing changes with the default configuration.
• If save_limit is 0, and save_limit_per_group is greater than zero, save_limit_per_group takes effect.
• If both are greater than zero, both take effect at the same time, deleting old results whenever either save_limit or save_limit_per_group is exceeded.

Open points:

• This considers tasks with group=None to be one group as well.

If this is something you want, I'll also add relevant test cases and documentation.

[codecov-io]

Codecov Report

Merging #508 (73e2278) into master (e258005) will decrease coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #508      +/-   ##
==========================================
- Coverage   90.54%   90.52%   -0.03%     
==========================================
  Files          47       47              
  Lines        3088     3091       +3     
==========================================
+ Hits         2796     2798       +2     
- Misses        292      293       +1     

Impacted Files	Coverage Δ
django_q/cluster.py	91.55% <50.00%> (-0.19%)	⬇️
django_q/conf.py	73.68% <100.00%> (+0.23%)	⬆️

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[Koed00]

Before I merge this, could you add a small entry in the documentation for this new option? Otherwise people will never find it.
Thanks for the work!