* dns: disable cache with "cache_size: 0" setting

Close #1591

Squashed commit of the following:

commit 547a971
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Tue Jun 16 14:46:06 2020 +0300

    * dns: disable cache with "cache_size: 0" setting

dnsforward/config.go

@@ -138,8 +138,6 @@ func (s *Server) createProxyConfig() (proxy.Config, error) {
 		Ratelimit:              int(s.conf.Ratelimit),
 		RatelimitWhitelist:     s.conf.RatelimitWhitelist,
 		RefuseAny:              s.conf.RefuseAny,
-		CacheEnabled:           true,
-		CacheSizeBytes:         int(s.conf.CacheSize),
 		CacheMinTTL:            s.conf.CacheMinTTL,
 		CacheMaxTTL:            s.conf.CacheMaxTTL,
 		UpstreamConfig:         s.conf.UpstreamConfig,
@@ -148,6 +146,11 @@ func (s *Server) createProxyConfig() (proxy.Config, error) {
 		EnableEDNSClientSubnet: s.conf.EnableEDNSClientSubnet,
 	}
 
+	if s.conf.CacheSize != 0 {
+		proxyConfig.CacheEnabled = true
+		proxyConfig.CacheSizeBytes = int(s.conf.CacheSize)
+	}
+
 	proxyConfig.UpstreamMode = proxy.UModeLoadBalance
 	if s.conf.AllServers {
 		proxyConfig.UpstreamMode = proxy.UModeParallel