Add support for read concern

[abarto]

Adds support for setting the read concern at the collection level restoring compatibility with mongoengine 0.20.

Fixes: #625

[codecov]

Codecov Report

Merging #628 into develop will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #628      +/-   ##
===========================================
+ Coverage    94.76%   94.82%   +0.06%     
===========================================
  Files           16       17       +1     
  Lines         3247     3267      +20     
===========================================
+ Hits          3077     3098      +21     
+ Misses         170      169       -1     

Impacted Files	Coverage Δ
mongomock/database.py	99.23% <ø> (ø)
mongomock/collection.py	91.71% <100.00%> (+0.02%)	⬆️
mongomock/read_concern.py	100.00% <100.00%> (ø)
mongomock/filtering.py	98.92% <0.00%> (+0.35%)	⬆️

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[pcorpet]

Thanks a lot for your contribution. Please address my comments and try to fix the CI

[lsmar]

Any plans to integrate this into a release? 3.19.1?