Support access format for database notification targets (#3953)

* Add configuration parameter "format" for db targets and perform
  configuration migration.
* Add PostgreSQL `access` format: This causes Minio to append all events
  to the configured table. Prefix, suffix and event filters continue
  to be supported for this mode too.
* Update documentation for PostgreSQL notification target.
* Add MySQL `access` format: It is very similar to the same format for
  PostgreSQL.
* Update MySQL notification documentation.

cmd/notify-elasticsearch.go

@@ -19,6 +19,7 @@ package cmd
 import (
 	"encoding/hex"
 	"errors"
+	"fmt"
 	"io/ioutil"
 
 	"github.com/Sirupsen/logrus"
@@ -29,6 +30,7 @@ import (
 // elasticQueue is a elasticsearch event notification queue.
 type elasticSearchNotify struct {
 	Enable bool   `json:"enable"`
+	Format string `json:"format"`
 	URL    string `json:"url"`
 	Index  string `json:"index"`
 }
@@ -37,6 +39,11 @@ func (e *elasticSearchNotify) Validate() error {
 	if !e.Enable {
 		return nil
 	}
+	if e.Format != formatNamespace {
+		return fmt.Errorf(
+			"Elasticsearch Notifier Error: \"format\" must be \"%s\"",
+			formatNamespace)
+	}
 	if _, err := checkURL(e.URL); err != nil {
 		return err
 	}