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/config-old.go

@@ -413,3 +413,20 @@ type serverConfigV15 struct {
 	// Notification queue configuration.
 	Notify *notifier `json:"notify"`
 }
+
+// serverConfigV16 server configuration version '16' which is like
+// version '15' except it makes a change to logging configuration.
+type serverConfigV16 struct {
+	Version string `json:"version"`
+
+	// S3 API configuration.
+	Credential credential  `json:"credential"`
+	Region     string      `json:"region"`
+	Browser    BrowserFlag `json:"browser"`
+
+	// Additional error logging configuration.
+	Logger *loggers `json:"logger"`
+
+	// Notification queue configuration.
+	Notify *notifier `json:"notify"`
+}