Transmit config to new worker with a protobuf type + protobuf channels

[Keksoj]

When a new worker is created, the whole Config object is passed to it:

sozu/bin/src/worker.rs

Lines 340 to 342 in bc49d7f

	main_to_worker_channel
	.write_message(config)
	.with_context(|| "Could not send config to the new worker using the channel")?;

This Config object contains necessary infos, like timeouts, but also proxy business infos like clusters, frontends etc. that are not used directly and are re-sent to the new worker later on in a different form.

Since we strive to have a protobuf-only communication both without and within Sōzu,
and since the Config object can be translated to protobuf only with difficulty, and no real purpose (as most of it is unused),
this PR changes the passing of the Config object to a ServerConfig. This struct was already derived from Config, was present in the Sōzu lib, but now:

• it is completed with necessary fields (buffer sizes, etc)
• it is written in protobuf
• it is passed to a new worker when created

This PR paves the way to changing the Sōzu channels in order to stop serializing structs to JSON, but to make use of the protobuf binary serialization.

One thing remains in a JSON form, though: requests written to a state file and passed to sōzu using a LoadState message.

[Wonshtrum]

Looks good. Great update!

[Keksoj]

We merged #1001, a unique commit, into this branch, here is the abstract of this PR:

This is the last step in the implementation of protobuf in Sōzu: use protobuf for all communications of Sōzu, within and without. See #1000

Coded with @Wonshtrum , this PR:

• makes channel transmit protobuf messages
• in a binary serialization
• with a fixed-size delimiter that tells how long the message is

Here is a simple benchmark of performance improvements:

We will serialize, write in a buffer, read from the buffer and deserialize, a 1000000 times, this object:
RequestHttpFrontend { cluster_id: Some("cluster_1"), address: "127.0.0.1:8080", hostname: "lolcatho.st", path: PathRule { kind: Prefix, value: "/" }, method: Some("GET"), position: Tree, tags: {"app_id": "a782f2e4-ee42-4384-9757-2b13d4198105", "owner": "Clever Cloud"} }
serde_json serialized it in 216 bytes
Serde JSON needed 23.838388363s
prost serialized it in 125 bytes
Prost needed 12.118231414s