Switch the usage of `UseIn` about tusd

[Acconut]

When I first designed the StoreComposer API, I used a similar approach as you are proposing here but it had its flaws. First of all, it does not properly work if you have a struct embedding a DataStore as it will hide all additional interfaces which are implemented by the embedded struct. Here is an example to help you understand this issue: https://play.golang.org/p/Eih7d2FvqH. This behavior is un-intuitive and I was very surprised when I first encountered it. Therefore I would prefer to save people the time of catching such a bug.

Secondly, I like my code explicit. It is way more descriptive to tell exactly which extension should be used in a given composer. It makes reading, understanding and debugging easier and that's even worth the additional lines which have to be written.

Finally, a StoreComposer will be created using exactly this method (as you described) when you pass a DataStore instead of an StoreComposer in the configuration (see https://github.com/tus/tusd/blob/master/config.go#L65-L68 and https://github.com/tus/tusd/blob/master/composer.go#L29-L50). This code is not there because I like it but simply to keep it backwards-compatible because the StoreComposer was added later as I discovered weird behavior with interfaces as I described in the first paragraph.

All in all I am not an eager fan of this idea but I am looking forward to your answer.

[oliverpool]

The main goal is to simplify the example of the Readme:

    // A storage backend for tusd may consist of multiple different parts which
    // handle upload creation, locking, termination and so on. The composer is a
    // place where all those seperated pieces are joined together. In this example
    // we only use the file store but you may plug in multiple.
    composer := tusd.NewStoreComposer()
    compoer.UseAllExtensions(store) // instead of store.UseIn(composer)

If you think it might be a good idea, I can make a PR with this.

[oliverpool]

I saw your example and didn't know about this behavior: thanks for pointing it out!

Maybe you could add a link to your example (or this issue) in the source so that it appears in the godoc

    // The usage of this field is deprecated and should be avoided in favor of
    // StoreComposer (see https://github.com/tus/tusd/issues/76).