ngclient: make update progress easier to follow in logs

[jku]

This might be a hard issue to close but I'd like it if it was easier to follow the client update process in the logs:
It's certainly already possible with debug logging but requires a lot of reading and probably a fairly deep understanding of the components.

I'd like to see

• the major specification steps (the ones required to see which branches the code took in validating metadata)
• State changes in "trusted metadata set" with
  • local metadata loads
  • metadata downloads

...without any other logs getting in the way

This is specifically not a 1.0 blocker: just moving a TODO item from code to issue tracker