The Quic Network Simulator `handshakeloss` testcase instructs the `neqo-client`
to download from multiple URLs in series. The client takes one URL after the
next and tries to request it:

``` rust
to_request = &remaining[..1];
remaining = &remaining[1..];
```

This will panic on the last URL, trying to access the first element in an empty
slice.

This commit removes the out-of-range access. Instead of using slice operations
which hide potential panics, it uses owned collections (`VecDeque` and `Vec`)
and their safe methods (e.g. `pop_front`, `drain`).

…t-of-bound