Update all plugins to use plugin/pkg/log (#1694)

* Update all plugins to use plugin/pkg/log

I wish this could have been done with sed. Alas manually changed all
callers to use the new plugin/pkg/log package.

* Error -> Info

* Add docs to debug plugin as well

plugin/proxy/healthcheck_test.go

@@ -2,8 +2,6 @@ package proxy
 
 import (
 	"fmt"
-	"io/ioutil"
-	"log"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -18,10 +16,6 @@ import (
 	"github.com/miekg/dns"
 )
 
-func init() {
-	log.SetOutput(ioutil.Discard)
-}
-
 func TestUnhealthy(t *testing.T) {
 	// High HC interval, we want to test the HC after failed queries.
 	config := "proxy . %s {\n health_check /healthcheck:%s 10s \nfail_timeout 100ms\n}"