Final changes to make all code follow our async conventions

[albrow]

Fixes #96.

Depends on #307. After that PR is merged I'll change the base branch for this one to development.

Summary of changes:

• Make core.App.Start blocking.
• Remove core.App.Stop.
• Avoid log.Panic and log.Fatal whenever possible (there are some lingering cases where e.g., we pass a function into another function and don't have a good way to return errors).
• Remove the blockwatch.Watcher.Errors channel. For non-critical errors, we log them in blockwatch.Watcher directly. For critical errors, we return them in blockwatch.Watcher.Watch. This makes blockwatch more aligned with the rest of our async code.
• Don't start any goroutines without a way to stop them (usually by waiting for ctx.Done() but sometimes with short timeouts or other signals).