fix(ext/flash): graceful server startup/shutdown with unsettled promises in mind (#16616)

This PR resets the revert commit made by #16610, bringing back #16383
which attempts to fix the issue happening when we use the flash server
with `--watch` option enabled.
Also, some code changes are made to pass the regression test added in
#16610.

ext/flash/socket.rs

@@ -1,23 +1,26 @@
 use deno_core::error::AnyError;
 use mio::net::TcpStream;
-use std::{
-  cell::UnsafeCell,
-  future::Future,
-  io::{Read, Write},
-  pin::Pin,
-  sync::{Arc, Mutex},
-};
+use std::cell::UnsafeCell;
+use std::future::Future;
+use std::io::Read;
+use std::io::Write;
+use std::marker::PhantomPinned;
+use std::pin::Pin;
+use std::sync::Arc;
+use std::sync::Mutex;
 use tokio::sync::mpsc;
 
 use crate::ParseStatus;
 
 type TlsTcpStream = rustls::StreamOwned<rustls::ServerConnection, TcpStream>;
 
+#[derive(Debug)]
 pub enum InnerStream {
   Tcp(TcpStream),
   Tls(Box<TlsTcpStream>),
 }
 
+#[derive(Debug)]
 pub struct Stream {
   pub inner: InnerStream,
   pub detached: bool,
@@ -26,6 +29,7 @@ pub struct Stream {
   pub parse_done: ParseStatus,
   pub buffer: UnsafeCell<Vec<u8>>,
   pub read_lock: Arc<Mutex<()>>,
+  pub _pinned: PhantomPinned,
 }
 
 impl Stream {