abstract_table_generator.cpp: Clean up spaghetti code

[mrks]

That code is part of a lambda. We return from the lambda, not from the entire method.

I guess it is partially my fault for just wrapping the lambda around the code to make it multi-threaded. We should probably move the inner part of the lambdas (in the following steps, too) into their own functions to clear this up a bit.

Originally posted by @mrks in https://github.com/hyrise/hyrise/issues/2297#issuecomment-757456188