devp2p: Move UDP socket handling from Discovery to Host. (#8790)

* devp2p: Move UDP socket handling from Discovery to Host.

* devp2p: Fix bug with potentially incorrect UDP registration.

This works right now because the Host handler happens to be the first one
registered on the IoService.

* devp2p: Use 0-initialized memory buffer instead of unsafe.

* Remove send_queue field from public interface of Discovery.

* Rename Datagramm to Datagram.

sed -i 's/Datagramm/Datagram/g' util/network-devp2p/src/discovery.rs util/network-devp2p/src/host.rs
sed -i 's/datagramm/datagram/g' util/network-devp2p/src/discovery.rs util/network-devp2p/src/host.rs

* Include target in log statements.