This is something we've been working in experimental-client branch so far and would like to merge to develop now. It's a new client library implementation:

• Uses Metadata API
• Is partly based on current tuf/client/, but no longer shares any modules with it (except exceptions)
• There's a major simplification in the way downloads are handled due to removing mirrors support
• Tracking of valid metadata has been separated into another component
• There's no MultiRepoUpdater or mirrors support

The implementation is roughly feature complete and ready for a test-drive.

There are still TODO-items that we'd like to handle before this could replace the current client, but we feel this would be a good time to merge so future work can be shared and is visible to everyone. Teodora has promised to post a list of things currently on our TODO-list here.

On the PR itself:

• Review is very welcome, but I hope we can consider ngclient directory a work-in-progress, in that we can file issues instead of fixing all review items here
• I've decided to squash the commits in this branch (the content matches experimental-client) The reasons were
  • there were a number of file moves while we were deciding what to do
  • we made decisions that we decided to back away from in the branch (but due to other changes did not use revert) so quite a few commits are just red herrings
  • there are ~100 commits and multiple merges of develop in experimental-client so history becomes a little hard to rack

If others would rather see the full history, I can make this PR from the experimental-client branch too.