feat: Add Downloader::with_pool_options() constructor (#213)

Adds a `Downloader::with_pool_options()` constructor that allows
configuring the internal `ConnectionPool` options.

Currently `Downloader::new()` hardcodes `Default::default()` for pool
options, which means the `idle_timeout` is always 5 seconds. The
`ConnectionPool` and `Options` types are already public, but there's no
way to pass them through to the `Downloader`.


We're from the
[Psyche/Nousnet](https://github.com/PsycheFoundation/nousnet) team
In our use case, peers exchange gradient via iroh-blobs. With the 5s
idle timeout, every connection gets dropped between transfers and a new
one is created for the next round.

On a devnet run with 6 peers, over 8 hours we see:

```
MaxPathIdReached warnings: 3,098
NAT traversal warnings:    3,344
Connection open/close cycles per hour: ~1,300
Handshake aborts: 31
```

```
2025-03-09T10:15:02 WARN iroh: MaxPathIdReached for connection ...
2025-03-09T10:15:02 WARN iroh: NAT traversal to ... via ... failed
2025-03-09T10:15:03 new connection to peer ...
2025-03-09T10:15:25 connection closed (idle timeout)
2025-03-09T10:15:25 new connection to peer ...  <- same peer, 22s later
```

The `MaxPathIdReached` happens because QUIC path IDs are monotonically
increasing and never reused, constant connection churn burns through
them.

Being able to set a longer idle timeout (e.g. 60s) would let the pool
reuse connections across transfers, eliminating the churn entirely.


Here is how we plan to use it on NousNet
https://github.com/PsycheFoundation/nousnet/pull/600/changes

Feedback is really appreciated

---------

Signed-off-by: pefontana <fontana.pedro93@gmail.com>
Co-authored-by: pefontana <pedro@nous.com>

Author: pefontana

src/api/downloader.rs

@@ -64,10 +64,14 @@ pub enum DownloadProgressItem {
 }
 
 impl DownloaderActor {
-    fn new(store: Store, endpoint: Endpoint) -> Self {
+    fn new_with_opts(
+        store: Store,
+        endpoint: Endpoint,
+        pool_options: crate::util::connection_pool::Options,
+    ) -> Self {
         Self {
             store,
-            pool: ConnectionPool::new(endpoint, crate::ALPN, Default::default()),
+            pool: ConnectionPool::new(endpoint, crate::ALPN, pool_options),
             tasks: JoinSet::new(),
             running: HashSet::new(),
         }
@@ -341,8 +345,16 @@ impl IntoFuture for DownloadProgress {
 
 impl Downloader {
     pub fn new(store: &Store, endpoint: &Endpoint) -> Self {
+        Self::new_with_opts(store, endpoint, Default::default())
+    }
+
+    pub fn new_with_opts(
+        store: &Store,
+        endpoint: &Endpoint,
+        pool_options: crate::util::connection_pool::Options,
+    ) -> Self {
         let (tx, rx) = tokio::sync::mpsc::channel::<SwarmMsg>(32);
-        let actor = DownloaderActor::new(store.clone(), endpoint.clone());
+        let actor = DownloaderActor::new_with_opts(store.clone(), endpoint.clone(), pool_options);
         n0_future::task::spawn(actor.run(rx));
         Self { client: tx.into() }
     }