Better fix for clang errors and reduced warnings

[neworderofjamie]

The better fix for the actual compiler error was to add trailing return types -> uint64_t to the lambda functions.

However, the majority of the changes are to fix warnings, mostly:

• Unused captured lambda parameters
• Missing virtual destructors in a few base classes
• A few unused variables of other sorts and a whole unused method

[tnowotny]

Looks good and fix of the actual error is general (not Mac-specific) - nice.

[neworderofjamie]

I would suspect it will fix compilation with clang on other platforms as well (and reduce the warnings)