Refactore/improve secrets code

This is an improvement/refactor of secrets management:

• modularization of encryption, storage and bootstrap/access of secrets.
• make it more explicit how we use secrets. Previous code made use of indexes to access parts of a larger blob and that was horrible to read. I did not change how secrets are used, so this might still be a bit ugly, but at least it is now explicit what we do, and easier to change.
• change the representation in secrets.json file. Instead of having one larger blob, i now use a dictionary so there's never a need to slice.