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/notifiers.go

@@ -43,6 +43,10 @@ const (
 	queueTypeKafka = "kafka"
 	// Static string for Webhooks
 	queueTypeWebhook = "webhook"
+
+	// Notifier format value constants
+	formatNamespace = "namespace"
+	formatAccess    = "access"
 )
 
 // Topic type.