Download missing directory information and store it to disk
We'll need to have code that does more or less the following:
- Decide which resources we are missing, and need to download
- Batch them into requests
- Make those requests via HTTP-over-begindir
- Receive the result
- Decompress the result
- Validate the result
- Store the result to disk if it is valid.
- Find a way to let the in-memory directory exist in an 'updating' state.
- Update the in-memory directory with the result.