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-redis.go

@@ -17,6 +17,7 @@
 package cmd
 
 import (
+	"fmt"
 	"io/ioutil"
 	"time"
 
@@ -27,6 +28,7 @@ import (
 // redisNotify to send logs to Redis server
 type redisNotify struct {
 	Enable   bool   `json:"enable"`
+	Format   string `json:"format"`
 	Addr     string `json:"address"`
 	Password string `json:"password"`
 	Key      string `json:"key"`
@@ -36,6 +38,11 @@ func (r *redisNotify) Validate() error {
 	if !r.Enable {
 		return nil
 	}
+	if r.Format != formatNamespace {
+		return fmt.Errorf(
+			"Redis Notifier Error: \"format\" must be \"%s\"",
+			formatNamespace)
+	}
 	if _, err := checkURL(r.Addr); err != nil {
 		return err
 	}