Don't leak goroutines via mDNS queries

mDNSConn.Query() will block indefinitely until something replies. Much
of the time, nothing will, and the goroutines making these queries will
block forever. By passing the CandidateHost's context, the queries will
at least be cancelled when the host disconnects.

README.md

@@ -55,6 +55,7 @@ Check out the **[contributing wiki](https://github.com/pion/webrtc/wiki/Contribu
 * [BUPTCZQ](https://github.com/buptczq)
 * [Henry](https://github.com/cryptix)
 * [Jerko Steiner](https://github.com/jeremija)
+* [Sidney San Martín](https://github.com/s4y)
 
 ### License
 MIT License - see [LICENSE](LICENSE) for full text

agent.go

@@ -741,7 +741,7 @@ func (a *Agent) resolveAndAddMulticastCandidate(c *CandidateHost) {
 	if a.mDNSConn == nil {
 		return
 	}
-	_, src, err := a.mDNSConn.Query(context.TODO(), c.Address())
+	_, src, err := a.mDNSConn.Query(c.context(), c.Address())
 	if err != nil {
 		a.log.Warnf("Failed to discover mDNS candidate %s: %v", c.Address(), err)
 		return