Refactor uploaders to use different upload strategies

Previously it was particularly tricky to add a new object storage method
because you had to be aware of how to deal with different Goroutines and
contexts to handle the Workhorse upload flow
(https://docs.gitlab.com/ee/development/uploads.html#direct-upload). In
addition, the execution engine to handle this was duplicated across
multiple files. The execution engine essentially did the following:

1. Set up an upload context with a deadline
2. Record upload metrics
3. Initialize cleanup functions
4. Initiate upload
5. Validate upload ETag
6. Do cleanup (e.g. delete the temporary file)

To reduce code duplication and to make it easier to add new object
stores, the common execution sequence is now encapsulated in the
`uploader` `Execute()` method. We also introduce an `UploadStrategy`
interface that handles the details of the uploads, and `Execute()` calls
methods on this interface.

Now adding a new object storage type is a matter of implementing the
`UploadStrategy` interface without needing to understand the details of
the execution engine.