Previously the `neqo-bin` server would read a set of
datagrams from the socket and allocate them:

``` rust
let dgrams: Vec<Datagram> = dgrams.map(|d| d.to_owned()).collect();
```

This was done out of convenience, as handling `Datagram<&[u8]>`s, each borrowing
from `self.recv_buf`, is hard to get right across multiple `&mut self`
functions, that is here `self.run`, `self.process` and `self.find_socket`.

This commit combines `self.process` and `self.find_socket` and passes a socket
index, instead of the read `Datagram`s from `self.run` to `self.process`, thus
making the Rust borrow checker happy to handle borrowing `Datagram<&[u8]>`s
instead of owning `Datagram`s.

…-alloc