This PR makes two improvements to Collection::restore_shard_snapshot:

First, it makes it cancel-safe, so that it's a bit safer to use. I don't think we have any issues with this method as-is, but it's just nice to have.

Second, restructures restore_shard_snapshot as a "two-step" future. It now returns additional future. E.g.:

// `restore_shard_snapshot` is `async fn`, that returns a future, that perfoms *validation* of snapshot archive
let validate = collection.restore_shard_snapshot(...);

// when you `await` it, it returns *another* future, that performs snapshot restore *in background*
// (we can await first future to validate snapshot with `wait=false`, and return an error *before* returning `accepted`)
let restore = validate.await?;

// and you can await this future *if you want to* to get result of recovery
// (we can await this to return response with `wait=true`)
let result = restore.await?;

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