eth/protocols/eth: prevent hanging dispatch (#30918)

This PR attempts to fix a strange test-failure (timeout) observed on a
windows-32 platform.

https://ci.appveyor.com/project/ethereum/go-ethereum/builds/51174391/job/d8ascanwwltrlqd5

A goroutine is stuck trying to deliver a response:
```
goroutine 9632 [select, 29 minutes]:
github.com/ethereum/go-ethereum/eth/protocols/eth.(*Peer).dispatchResponse(0x314f100, 0x3e5f6d0, 0x3acbb84)
	C:/projects/go-ethereum/eth/protocols/eth/dispatcher.go:172 +0x2a5
github.com/ethereum/go-ethereum/eth/protocols/eth.handleBlockHeaders({0x12abe68, 0x30021b8}, {0x12a815c, 0x40b41c0}, 0x314f100)
	C:/projects/go-ethereum/eth/protocols/eth/handlers.go:301 +0x173
github.com/ethereum/go-ethereum/eth/protocols/eth.handleMessage({0x12abe68, 0x30021b8}, 0x314f100)
	C:/projects/go-ethereum/eth/protocols/eth/handler.go:205 +0x4f6
github.com/ethereum/go-ethereum/eth/protocols/eth.Handle({0x12abe68, 0x30021b8}, 0x314f100)
	C:/projects/go-ethereum/eth/protocols/eth/handler.go:149 +0x33
github.com/ethereum/go-ethereum/eth.testSnapSyncDisabling.func1(0x314f100)
	C:/projects/go-ethereum/eth/sync_test.go:65 +0x33
github.com/ethereum/go-ethereum/eth.(*handler).runEthPeer(0x30021b8, 0x314f100, 0x427f648)
	C:/projects/go-ethereum/eth/handler.go:355 +0xe65
created by github.com/ethereum/go-ethereum/eth.testSnapSyncDisabling in goroutine 11
	C:/projects/go-ethereum/eth/sync_test.go:64 +0x54f
FAIL	github.com/ethereum/go-ethereum/eth	1800.138s

```

---------

Co-authored-by: Gary Rong <[email protected]>

eth/protocols/eth/dispatcher.go

@@ -174,6 +174,8 @@ func (p *Peer) dispatchResponse(res *Response, metadata func() interface{}) erro
 				return <-res.Done // Response delivered, return any errors
 			case <-res.Req.cancel:
 				return nil // Request cancelled, silently discard response
+			case <-p.term:
+				return errDisconnected
 			}
 		}
 

eth/sync_test.go

@@ -88,7 +88,7 @@ func testSnapSyncDisabling(t *testing.T, ethVer uint, snapVer uint) {
 	if err := empty.handler.downloader.BeaconSync(ethconfig.SnapSync, full.chain.CurrentBlock(), nil); err != nil {
 		t.Fatal("sync failed:", err)
 	}
-	empty.handler.enableSyncedFeatures()
+	time.Sleep(time.Second * 5) // Downloader internally has to wait a timer (3s) to be expired before exiting
 
 	if empty.handler.snapSync.Load() {
 		t.Fatalf("snap sync not disabled after successful synchronisation")