Early data clarity

[gretchenfrage]

I had a hard time getting 0-RTT actually working in practice. It escalated to me forking rustls so I could put in a bunch of print statements. In hopes of improving understandability and discoverability of this, this PR demonstrates the usage of 0-RTT in the client/server example, adds other features to the client/server example to facilitate 0-RTT testing, and adds and rewrites some of the doc comments. It does not actually change any quinn code or APIs.

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Tests with emulated localhost latency set to 100 ms.

Baseline no-0RTT test

cargo run --example server ./
cargo run --example client -- https://localhost:4433/Cargo.toml --suppress
# > connecting to localhost at [::1]:4433
# > connected at 205.968523ms
# > first response byte at 407.806377ms
# > response received in 17.335µs - 13238.661 KiB/s

Basic 0RTT test

cargo run --example server ./ --0rtt
cargo run --example client -- https://localhost:4433/Cargo.toml https://localhost:4433/Cargo.toml \
	--suppress --0rtt
# > connecting to localhost at [::1]:4433
# > client rejected 0rtt conversion
# > connected at 204.259169ms
# > first response byte at 406.096949ms
# > response received in 17.567µs - 13063.824 KiB/s
# >
# > connecting to localhost at [::1]:4433
# > client accepted 0rtt conversion
# > server accepted 0rtt data at 203.783772ms
# > first response byte at 203.80829ms
# > response received in 14.531µs - 15793.283 KiB/s

Test: 0-RTT, but not 0.5-RTT

cargo run --example server ./
cargo run --example client -- https://localhost:4433/Cargo.toml https://localhost:4433/Cargo.toml \
	--suppress --0rtt
# > connecting to localhost at [::1]:4433
# > client rejected 0rtt conversion
# > connected at 204.57444ms
# > first response byte at 406.396568ms
# > response received in 18.739µs - 12246.769 KiB/s
# >
# > connecting to localhost at [::1]:4433
# > client accepted 0rtt conversion
# > server accepted 0rtt data at 203.518732ms
# > first response byte at 405.201436ms
# > response received in 15.87µs - 14460.755 KiB/s

Test: 0.5-RTT, but not 0-RTT

cargo run --example server ./ --0rtt
cargo run --example client -- https://localhost:4433/Cargo.toml --suppress
# > connecting to localhost at [::1]:4433
# > connected at 206.004094ms
# > first response byte at 407.890297ms
# > response received in 21.695µs - 10578.114 KiB/s

Test: 0-RTT rejected by server

cargo run --example server ./ --0rtt
cargo run --example client -- https://localhost:4433/Cargo.toml https://localhost:4433/Cargo.toml \
	--suppress --0rtt --stdin
# > connecting to localhost at [::1]:4433
# > client rejected 0rtt conversion
# > connected at 204.058676ms
# > first response byte at 405.903475ms
# > response received in 20.768µs - 11050.279 KiB/s
# >
# > connecting to localhost at [::1]:4433
# > client accepted 0rtt conversion
# > server accepted 0rtt data at 203.678514ms
# > first response byte at 203.706407ms
# > response received in 12.453µs - 18428.668 KiB/s

# Now I Ctrl+C the server process
# Now I start it again:
cargo run --example server ./ --0rtt

# Now I input into the client process:
# https://localhost:4433/Cargo.toml

# > connecting to localhost at [::1]:4433
# > client accepted 0rtt conversion
# > server rejected 0rtt data at 204.140098ms
# > first response byte at 405.921651ms
# > response received in 16.168µs - 14194.223 KiB/s

# Now I input into the client process:
# https://localhost:4433/Cargo.toml

# connecting to localhost at [::1]:4433
# client accepted 0rtt conversion
# first response byte at 204.068826ms
# response received in 26.044µs - 8811.711 KiB/s