Code refactoring and cleanup

[enwi]

I have made some changes that fix some compiler errors as well as cleanup the code.

• Fixes comparison of an address with nullptr
• Add several missing return statements
• Add crude source code documentation
• Reorder cpp code according to header file
• Use have_keys of root class instead of redefining it
• Add helper class for automatically closing storage handle

[rgot-org]

Hello Moritz, thank you for your work ! Francois Riotte

[enwi]

I have some more improvements in the pipeline, MR coming later today :)