all: add configuration to limit max concurrent requests

Currently, there's no upper bound for how many requests that ctrld will
handle at a time. This could be problem on some low capacity routers,
where CPU/RAM is very limited.

This commit adds a configuration to limit how many requests that will be
handled concurrently. The default is 256, which should works well for
most routers (the default concurrent requests of dnsmasq is 150).

config_test.go

@@ -75,6 +75,7 @@ func TestConfigValidation(t *testing.T) {
 		{"os upstream", configWithOsUpstream(t), false},
 		{"invalid rules", configWithInvalidRules(t), true},
 		{"invalid dns rcodes", configWithInvalidRcodes(t), true},
+		{"invalid max concurrent requests", configWithInvalidMaxConcurrentRequests(t), true},
 	}
 
 	for _, tc := range tests {
@@ -176,3 +177,10 @@ func configWithInvalidRcodes(t *testing.T) *ctrld.Config {
 	}
 	return cfg
 }
+
+func configWithInvalidMaxConcurrentRequests(t *testing.T) *ctrld.Config {
+	cfg := defaultConfig(t)
+	n := -1
+	cfg.Service.MaxConcurrentRequests = &n
+	return cfg
+}