Use INSERT ... ON DUPLICATE KEY instead of REPLACE

There are sources on the Internet that indicate that this should be faster.

storage/mysql/tree_storage.go

@@ -34,7 +34,7 @@ import (
 
 // These statements are fixed
 const (
-	insertSubtreeMultiSQL = `REPLACE INTO Subtree(TreeId, SubtreeId, Nodes, SubtreeRevision) ` + placeholderSQL
+	insertSubtreeMultiSQL = `INSERT INTO Subtree(TreeId, SubtreeId, Nodes, SubtreeRevision) ` + placeholderSQL + ` ON DUPLICATE KEY UPDATE Nodes=VALUES(Nodes)`
 	insertTreeHeadSQL     = `INSERT INTO TreeHead(TreeId,TreeHeadTimestamp,TreeSize,RootHash,TreeRevision,RootSignature)
 		 VALUES(?,?,?,?,?,?)`