fix(dialer): prevent hang on HTTP+ICMP by enabling QUIC keepalive by default

The ICMP dialer required explicit keepalive:true to send QUIC PING frames,
while the regular QUIC dialer enabled keepalive by default (10s period).
Without PING frames, quic-go's 30s idle timeout silently killed connections,
causing HTTP+ICMP tunnels to hang after periods of inactivity.

Fixes go-gost/gost#352

Also in this commit:
- Release session mutex before calling GetConn() in both ICMP and QUIC
  dialers, so a dead session doesn't block all Dial() calls while
  OpenStreamSync blocks
- Add 30s timeout context to OpenStreamSync to prevent indefinite blocking
  if the QUIC session dies between liveness check and stream open
- Guard session cache deletion with pointer identity check to avoid
  evicting a newly created session during a race

dialer/quic/conn.go

@@ -3,6 +3,7 @@ package quic
 import (
 	"context"
 	"net"
+	"time"
 
 	"github.com/quic-go/quic-go"
 )
@@ -12,7 +13,9 @@ type quicSession struct {
 }
 
 func (session *quicSession) GetConn() (*quicConn, error) {
-	stream, err := session.session.OpenStreamSync(context.Background())
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+	stream, err := session.session.OpenStreamSync(ctx)
 	if err != nil {
 		return nil, err
 	}