Add support for COUNT and fix a bug in the GROUP BY HashMap optimization

[kcaliban]

Add support for COUNT and prepare adding support for the other aggregate expressions in the hash map-based implementation of GROUP BY.

Also fix a critical bug when grouped variables occur in expressions.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (cdbe121) 84.20% compared to head (156a239) 84.26%.

Files	Patch %	Lines
src/engine/GroupBy.h	88.46%	0 Missing and 3 partials ⚠️
src/engine/GroupBy.cpp	99.26%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1222      +/-   ##
==========================================
+ Coverage   84.20%   84.26%   +0.05%     
==========================================
  Files         305      305              
  Lines       28777    28859      +82     
  Branches     3396     3408      +12     
==========================================
+ Hits        24233    24318      +85     
  Misses       3162     3162              
+ Partials     1382     1379       -3     

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[joka921]

Thank you very much, I have added some suggestions and comments.
Please also have a look at the sonarcloud suggestions, those are complimentary to my suggestions.

[joka921]

Almost done, this is already much better.

[joka921]

Thank you.
As soon as the checks have finished, I will merge this.

[sonarqubecloud]

Quality Gate passed

The SonarCloud Quality Gate passed, but some issues were introduced.

5 New issues
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[hannahbast]

@kcaliban @joka921 This does not compile!

[kcaliban]

@hannahbast @joka921 Fixed in #1224