Small autoupdate improvements

[RebeccaMahany]

Relates to #954.

This is the next step toward using our new TUF infrastructure: a couple small improvements, including addressing code review comments left on #1111 and others:

• Ensure that repeated errors when trying to autoupdate launcher will not prevent us from trying to update osquery
• Use a temp directory for staging downloads
• Add a lock to the library to ensure updates aren't running in multiple places for the same library

Subsequent PRs will tackle the following (order is not set in stone):

1. Trigger a reload/restart after a successful update; launcher init performs version selection using the new update library (potentially falling back to findNew); launcher performs library tidying after version selection
2. Other improvements/refactors (use knapsack/flags for configuration, ship a more up-to-date TUF repo)
3. An "update now" functionality tied to control server
4. Eventually removing the old notary autoupdater

Previous work:

1. Run new TUF autoupdater side-by-side with notary autoupdater #1081
2. Expose data and metrics about new TUF autoupdater #1103
3. Point to production TUF infra #1108
4. Perform retry on TUF update #1110
5. Add library manager to handle TUF downloads #1111

[seejdev]

The comments are helpful, thank you

[directionless]

I'm not tracking all the details, but it seems okay at a high level.