Allow overlapping Zones if binding addresses are different (#1530)

* add OverlapChecker, move the test of overlap AFTER the directive setup process, change key of configs to allow multiple same key

* glitch when rebase. init of Config should include the default host

* add tests for the registering of configuration
rename multicast in 'unbound'.
add comments on the validator

* - merged zoneAddr and addrKey that are very similar
- move maps of Validator to zoneAddr, avoinding need to have string representation of zoneaddr
- moving key build for saving Config at Config side instead of dnsContext

* - UT on saving config is now useless.

* - cannot cleanup access to Configs after setup. Deferred function to Start, use it

* - cleanup register unit tests. remove useless function

* - address comments of review. name of validator, comments, simplify registerAndCheck

* - fixes after review. renaming a function and a comment

core/dnsserver/config.go

@@ -2,6 +2,7 @@ package dnsserver
 
 import (
 	"crypto/tls"
+	"fmt"
 	"net"
 
 	"github.com/coredns/coredns/plugin"
@@ -68,16 +69,22 @@ func (c *Config) HostAddresses() string {
 	return all
 }
 
+// keyForConfig build a key for identifying the configs during setup time
+func keyForConfig(blocIndex int, blocKeyIndex int) string {
+	return fmt.Sprintf("%d:%d", blocIndex, blocKeyIndex)
+}
+
 // GetConfig gets the Config that corresponds to c.
 // If none exist nil is returned.
 func GetConfig(c *caddy.Controller) *Config {
 	ctx := c.Context().(*dnsContext)
-	if cfg, ok := ctx.keysToConfigs[c.Key]; ok {
+	key := keyForConfig(c.ServerBlockIndex, c.ServerBlockKeyIndex)
+	if cfg, ok := ctx.keysToConfigs[key]; ok {
 		return cfg
 	}
 	// we should only get here during tests because directive
 	// actions typically skip the server blocks where we make
 	// the configs.
-	ctx.saveConfig(c.Key, &Config{ListenHosts: []string{""}})
+	ctx.saveConfig(key, &Config{ListenHosts: []string{""}})
 	return GetConfig(c)
 }