SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.
Replaces them with forward declarations of used types, or removes them entirely if they aren't used at all. This also replaces certain Common headers with less inclusive ones (in terms of definitions they pull in).
Otherwise, it would work but any async sending would be delayed by 4ms or
wait until the next packet was received.
Also increase the client timeout to 250ms, since enet_host_service is now
really interrupted.
