- dns: limit the number of active goroutines for incoming requests processing

Close #2015

Squashed commit of the following:

commit 90ba06f1fce22a452b4d61db62bd950b976debd1
Merge: 9494b29b 473d8818
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Fri Aug 14 19:14:26 2020 +0300

    Merge remote-tracking branch 'origin/master' into max-go

commit 9494b29b65ae8fe593a487984bed051aa78e4ff9
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Fri Aug 14 17:03:00 2020 +0300

    + max_goroutines setting

commit 87071a5e0ed43be192a7755fb25764cd4519da5a
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Fri Aug 14 15:29:00 2020 +0300

    - dns: limit the number of active goroutines for incoming requests processing

dnsforward/dnsforward.go

@@ -176,6 +176,9 @@ func (s *Server) Prepare(config *ServerConfig) error {
 				return fmt.Errorf("DNS: invalid custom blocking IP address specified")
 			}
 		}
+		if s.conf.MaxGoroutines == 0 {
+			s.conf.MaxGoroutines = 50
+		}
 	}
 
 	// 2. Set default values in the case if nothing is configured