destroy_all should return to Ruby, delete_all happens in cypher

This has bugged me for a little while. Our `destroy_all` class method was behaving like `delete_all`, killing everything in Cypher. This fixes that, updates all specs that were using it for cleanup, and adds new specs to prove that it is calling `all` on the class, which we take to mean that it is returning them to Ruby.

I considered going a step further and proving that it is executing callbacks but that's not the point of that spec file and I wanted to keep it as basic as possible.