Provide more locking control for update process

[doanac]

The main problem is that users have no control over when containers are downloaded for update. There are two issues at play:

1. Containers aren't actually downloaded during the "fetch" phase. They get downloaded when we call docker-compose up.
2. We need a locking mechanism in place for this new step.

Our current lock in aktualizr-lite is just for the "update". We'll need a new CLI option for this "download" lock as well.

[doanac]

Here is my branch with the progress so far:

https://github.com/doanac/aktualizr/commits/docker-app-fetch

[doanac]

This has been merged. It will go out in our next v60 update.