BACKUP: track and report DDLs

On BACKUP START the server should start monitoring DDLs and collect all names of tables that had their metadata changed.

This can be collected in a temporary table (MEMORY with overflow to disk, as usual) and returned, for example, in a INFORMATION_SCHEMA table (preferrably with file names, if possible).

The backup client can thus retrieve all such tables (that were supposedly created/dropped/renamed/modified during the backup process) and copy them again (or drop or rename in the backup — the I_S table should have enough information for that).