Add support for MySQL notifications (fixes #3818) (#3907)

As a new configuration parameter is added, configuration version is
bumped up from 14 to 15.

The MySQL target's behaviour is identical to the PostgreSQL: rows are
deleted from the MySQL table on delete-object events, and are
created/updated on create/over-write events.

cmd/notifiers.go

@@ -37,6 +37,8 @@ const (
 	queueTypeRedis = "redis"
 	// Static string indicating queue type 'postgresql'.
 	queueTypePostgreSQL = "postgresql"
+	// Static string indicating queue type 'mysql'.
+	queueTypeMySQL = "mysql"
 	// Static string indicating queue type 'kafka'.
 	queueTypeKafka = "kafka"
 	// Static string for Webhooks
@@ -156,6 +158,24 @@ func isPostgreSQLQueue(sqsArn arnSQS) bool {
 	return true
 }
 
+// Returns true if queueArn is for MySQL.
+func isMySQLQueue(sqsArn arnSQS) bool {
+	if sqsArn.Type != queueTypeMySQL {
+		return false
+	}
+	msqlNotify := serverConfig.Notify.GetMySQLByID(sqsArn.AccountID)
+	if !msqlNotify.Enable {
+		return false
+	}
+	myC, err := dialMySQL(msqlNotify)
+	if err != nil {
+		errorIf(err, "Unable to connect to MySQL server %#v", msqlNotify)
+		return false
+	}
+	defer myC.Close()
+	return true
+}
+
 // Returns true if queueArn is for Kafka.
 func isKafkaQueue(sqsArn arnSQS) bool {
 	if sqsArn.Type != queueTypeKafka {