Use a single long-lived RPC connection when authenticated

The prior system spawned a new connection per request to enable parallelism,
yet kept hitting hyper::IncompleteMessages I couldn't track down. This
attempts to resolve those by a long-lived socket.

Halves the amount of requests per-authenticated RPC call, and accordingly is
likely still better overall.

I don't believe this is resolved yet but this is still worth pushing.

tests/full-stack/src/lib.rs

@@ -171,7 +171,7 @@ impl Handles {
     // If the RPC server has yet to start, sleep for up to 60s until it does
     for _ in 0 .. 60 {
       tokio::time::sleep(Duration::from_secs(1)).await;
-      let Ok(client) = HttpRpc::new(rpc.clone()) else { continue };
+      let Ok(client) = HttpRpc::new(rpc.clone()).await else { continue };
       if client.get_height().await.is_err() {
         continue;
       }

tests/full-stack/src/tests/mint_and_burn.rs

@@ -523,7 +523,7 @@ async fn mint_and_burn_test() {
           Coin::Monero,
           1_000_000_000_000,
           ExternalAddress::new(
-            serai_client::networks::monero::Address::new(monero_addr).unwrap().try_into().unwrap(),
+            serai_client::networks::monero::Address::new(monero_addr).unwrap().into(),
           )
           .unwrap(),
         )