Add perf_datagram_{server, client}

[aochagavia]

This PR adds a perf_datagram_server and perf_datagram_client to benchmark datagram transmit performance. There are two modes of operation: sending all the data through datagrams, or sending all the data in a single stream. That makes it possible to compare performance between the two. I dedided to add them as separate binaries because they differ too much from the existing perf_{client, server}, but I reused the code where possible.

Interestingly, there is a huge discrepancy in performance between datagrams and streams. Somehow the throughput when using datagrams is much lower! Below follows a comparison. I won't have time to investigate the reason of the difference, but as far as I can see the benchmarking code is correct.

Using datagrams

Server: cargo run --release --bin perf_datagram_server -- --no-protection

Client: cargo run --release --bin perf_datagram_client -- --no-protection

Output:

Transferred 2147484066 bytes in 6670 ms (307.04 MiB/s)
Bytes lost 2561048 (2.44 MiB)
Congestion events: 23
Congestion window: 454043

Using a stream

Server: cargo run --release --bin perf_datagram_server -- --no-protection --use-stream

Client: cargo run --release --bin perf_datagram_client -- --no-protection --use-stream

Output:

Transferred 2147483648 bytes in 1185 ms (1727.60 MiB/s)
Bytes lost 627092 (0.60 MiB)
Congestion events: 7
Congestion window: 4069176

[aochagavia]

Closing this, since there is little chance I'll be able to work on it anytime soon