my_myql: Handle deadlocks in mysql_stmt_execute

We've seen deadlocks when mxqkill races with job completion.

Handle deadlocks errors in mysql_stmt_execute. This is possible, because
we don't start multi-statement transactions.

mx_mysql.c

@@ -262,14 +262,13 @@ static int mx__mysql_stmt_bind_result(struct mx_mysql_stmt *stmt)
 
 static int mx__mysql_stmt_execute(struct mx_mysql_stmt *stmt)
 {
-    int res;
-
     assert(stmt);
     assert(stmt->stmt);
 
-    res = mysql_stmt_execute(stmt->stmt);
-    if (res == 0)
-        return 0;
+    do {
+        if (mysql_stmt_execute(stmt->stmt) == 0)
+            return 0;
+    } while (mysql_stmt_errno(stmt->stmt) == ER_LOCK_DEADLOCK);
 
     mx_mysql_save_error(mysql_stmt_error(stmt->stmt));