Make all fetch operations in fast-sync use the same retry-approach

There are a lot of different fetch operations going on in the fast sync process and even more are [on the way](https://github.com/casper-network/casper-node/pull/3282#discussion_r952961597).

We should unify all these operations to use the approach that leverages the `bootstrap_completed` flag maintained by `small_network`.

Summary of the current fetch operations is available here: https://github.com/casper-network/casper-node/issues/3275#issuecomment-1220763505