OnionAliasService should use threadsafe ISupports

OnionAliasService was born as a thread safe class (or so it should be: it has only one hashmap, and a mutex that protects the writings to it, but not the reads - it shouldn't be necessary, but I can lock it also when reading, if needed).

However, I should have used NS_DECL_THREADSAFE_ISUPPORTS, but I didn't.

So, reference counting may not be thread-safe, or so Firefox thinks, and crashes when compiled with debug enabled.