completely rewrite pq_proto (#12085)

libs/pqproto is designed for safekeeper/pageserver with maximum
throughput.

proxy only needs it for handshakes/authentication where throughput is
not a concern but memory efficiency is. For this reason, we switch to
using read_exact and only allocating as much memory as we need to.

All reads return a `&'a [u8]` instead of a `Bytes` because accidental
sharing of bytes can cause fragmentation. Returning the reference
enforces all callers only hold onto the bytes they absolutely need. For
example, before this change, `pqproto` was allocating 8KiB for the
initial read `BytesMut`, and proxy was holding the `Bytes` in the
`StartupMessageParams` for the entire connection through to passthrough.

proxy/src/serverless/sql_over_http.rs

@@ -17,7 +17,6 @@ use postgres_client::error::{DbError, ErrorPosition, SqlState};
 use postgres_client::{
     GenericClient, IsolationLevel, NoTls, ReadyForQueryStatus, RowStream, Transaction,
 };
-use pq_proto::StartupMessageParamsBuilder;
 use serde::Serialize;
 use serde_json::Value;
 use serde_json::value::RawValue;
@@ -41,6 +40,7 @@ use crate::context::RequestContext;
 use crate::error::{ErrorKind, ReportableError, UserFacingError};
 use crate::http::{ReadBodyError, read_body_with_limit};
 use crate::metrics::{HttpDirection, Metrics, SniGroup, SniKind};
+use crate::pqproto::StartupMessageParams;
 use crate::proxy::{NeonOptions, run_until_cancelled};
 use crate::serverless::backend::HttpConnError;
 use crate::types::{DbName, RoleName};
@@ -219,7 +219,7 @@ fn get_conn_info(
 
     let mut options = Option::None;
 
-    let mut params = StartupMessageParamsBuilder::default();
+    let mut params = StartupMessageParams::default();
     params.insert("user", &username);
     params.insert("database", &dbname);
     for (key, value) in pairs {