proxy: optimise future layout allocations (#12104)

A smaller version of #12066 that is somewhat easier to review.

Now that I've been using https://crates.io/crates/top-type-sizes I've
found a lot more of the low hanging fruit that can be tweaks to reduce
the memory usage.

Some context for the optimisations:

Rust's stack allocation in futures is quite naive. Stack variables, even
if moved, often still end up taking space in the future. Rearranging the
order in which variables are defined, and properly scoping them can go a
long way.

`async fn` and `async move {}` have a consequence that they always
duplicate the "upvars" (aka captures). All captures are permanently
allocated in the future, even if moved. We can be mindful when writing
futures to only capture as little as possible.

TlsStream is massive. Needs boxing so it doesn't contribute to the above
issue.

## Measurements from `top-type-sizes`:

### Before

```
10328 {async block@proxy::proxy::task_main::{closure#0}::{closure#0}} align=8
6120 {async fn body of proxy::proxy::handle_client<proxy::protocol2::ChainRW<tokio::net::TcpStream>>()} align=8
```

### After

```
4040 {async block@proxy::proxy::task_main::{closure#0}::{closure#0}}
4704 {async fn body of proxy::proxy::handle_client<proxy::protocol2::ChainRW<tokio::net::TcpStream>>()} align=8
```

proxy/src/pqproto.rs

@@ -186,7 +186,7 @@ where
 pub async fn read_message<'a, S>(
     stream: &mut S,
     buf: &'a mut Vec<u8>,
-    max: usize,
+    max: u32,
 ) -> io::Result<(u8, &'a mut [u8])>
 where
     S: AsyncRead + Unpin,
@@ -206,7 +206,7 @@ where
     let header = read!(stream => Header);
 
     // as described above, the length must be at least 4.
-    let Some(len) = (header.len.get() as usize).checked_sub(4) else {
+    let Some(len) = header.len.get().checked_sub(4) else {
         return Err(io::Error::other(format!(
             "invalid startup message length {}, must be at least 4.",
             header.len,
@@ -222,7 +222,7 @@ where
     }
 
     // read in our entire message.
-    buf.resize(len, 0);
+    buf.resize(len as usize, 0);
     stream.read_exact(buf).await?;
 
     Ok((header.tag, buf))