Logging of DDL statements during backup

Implement logging of DDL statements that happened between BACKUP STAGE start and BACKUP STAGE end.

The log will be written in data_dir/ddl.log. This file will be dropped (if exists) and created for each BACKUP START command.

The log file is a tab separated file with new line between commands.

The log format is:
datetime <tab> query <tab> original_storage_engine <tab> original_database <tab> original_table_name <tab> table_version <tab> new_storage_engine <tab> new_database <tab> new_table_name <tab> new_version_id <nl>

• The fields named new_... are only filled in case of rename or alter table rename
• table_version may be empty in case of very old .frm files (before MariaDB 10.0)
• original_storage_engine may be 'DATABASE' in case of CREATE/DROP database, 'VIEW' in case of CREATE/DROP VIEW or 'TRIGGER' in case of CREATE/DROP TRIGGER.
• original_table_name is empty in case of CREATE/DROP DATABASE.
• All table and database name are written in file name format (to ensure that there are no tabs in the names). This means that a table of name 't 1' is logged as 't@00201'. One can use the
  filename_to_tablename() and tablename_to_filename() functions to convert between table and file names.

The possible values for query are:

• ALTER
• BULK_INSERT
• CHANGE_INDEX
• CREATE ; The storage engine field contains either storage engine name, DATABASE, VIEW or TRIGGER
• DROP ; The storage engine field contains either storage engine name, DATABASE, VIEW or TRIGGER
• DROP_AFTER_CREATE
• RENAME
• TRUNCATE
• optimize
• repair