fix btree destructors #2127

[julienhenry]

Fixing destructor issues on the Btree datastructures.
Related to #2130 #2127

[codecov]

Codecov Report

Merging #2287 (8f30dd5) into master (89c831c) will increase coverage by 0.00%.
The diff coverage is 11.11%.

@@           Coverage Diff           @@
##           master    #2287   +/-   ##
=======================================
  Coverage   77.41%   77.42%           
=======================================
  Files         461      461           
  Lines       28996    29003    +7     
=======================================
+ Hits        22447    22455    +8     
+ Misses       6549     6548    -1     

Impacted Files	Coverage Δ
src/include/souffle/datastructure/BTreeDelete.h	56.85% <11.11%> (-0.59%)	⬇️
src/include/souffle/datastructure/BTree.h	98.31% <0.00%> (+0.42%)	⬆️
src/include/souffle/utility/ParallelUtil.h	85.27% <0.00%> (+0.77%)	⬆️
src/include/souffle/datastructure/UnionFind.h	97.80% <0.00%> (+2.19%)	⬆️
src/include/souffle/datastructure/LambdaBTree.h	75.57% <0.00%> (+2.29%)	⬆️

[b-scholz]

Unfortunately, a VTABLE (introducing a virtual destructor for class node) will slow us down - specifically for clearing memory. We would need some static means to fix this. We would need something like this:
https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern

[julienhenry]

Here is another attempt. Before calling delete, we check if this is an inner node or leaf node. If this is an inner node, make sure to reset its children to make sure they do not get deleted.

[b-scholz]

Thanks for fixing the bug!