Merge remote-tracking branch 'upstream/master'
Merge https://github.com/zanefisher/tuf into refactor-download-update
Merge branch 'refactor-download-update' of https://github.com/zanefisher/tuf into refactor-download-update
Merge with updater/download refactoring from @zanefisher.

Update download unit test to work after refactoring, but it is a little
incomplete (in particular, the unsafe_download function needs more testing).
Adapt the updater unit test against latest changes.
WIP on refactoring the updater and downloader.