Fix SQLite adapter with GroupedRecords

[yunzheng]

This PR ensures that we don't use record.__slots__ but rather rdict.keys() for creating the sqlite table. Small regression test is included.

Also fixes some other small issues I encountered with GroupedRecord class, mainly by renaming the class variables to dunder variables.

fixes #215

[codecov]

Codecov Report

❌ Patch coverage is 84.21053% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.21%. Comparing base (e9118cf) to head (2448a8c).

Files with missing lines	Patch %	Lines
flow/record/base.py	85.71%	2 Missing ⚠️
flow/record/selector.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #216      +/-   ##
==========================================
+ Coverage   84.18%   84.21%   +0.03%     
==========================================
  Files          35       35              
  Lines        3736     3737       +1     
==========================================
+ Hits         3145     3147       +2     
+ Misses        591      590       -1     

Flag	Coverage Δ
unittests	84.21% <84.21%> (+0.03%)	⬆️

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