The QUIC Interop Runner requires an http3 and http09 implementation for both
client and server. The client code is already structured into an http3 and an
http09 implementation since mozilla#1727.

This commit does the same for the server side, i.e. splits the http3 and http09
implementation into separate Rust modules.

There are two server implementations based on neqo:

1. https://github.com/mozilla/neqo/tree/main/neqo-bin/src/server
  - http3 and http09 implementation
  - used for manual testing and QUIC Interop

2. https://searchfox.org/mozilla-central/source/netwerk/test/http3server/src/main.rs
  - used to test Firefox

I assume one was once an exact copy of the other. Both implement their own I/O,
event loop, ... Since then, the two implementations diverged significantly.
Especially (1) saw a lot of improvements in recent months:

- mozilla#1564
- mozilla#1569
- mozilla#1578
- mozilla#1581
- mozilla#1604
- mozilla#1612
- mozilla#1676
- mozilla#1692
- mozilla#1707
- mozilla#1708
- mozilla#1727
- mozilla#1753
- mozilla#1756
- mozilla#1766
- mozilla#1772
- mozilla#1786
- mozilla#1787
- mozilla#1788
- mozilla#1794
- mozilla#1806
- mozilla#1808
- mozilla#1848
- mozilla#1866

At this point, bugs in (2) are hard to fix, see e.g.
mozilla#1801.

This commit merges (2) into (1), thus removing all duplicate logic and
having (2) benefit from all the recent improvements to (1).

…-central-http3-server

Nice to have. Adds multiple dependencies. Hard to justify for mozilla-central.