This worklog implements IMPORT from SDI files, i.e. for SEs that do not
implement their own SDI handling.

The FLUSH TABLES ... WITH READ LOCK statement combined with manual copy
of data, index and sdi files must be used to create a consistent snapshot.

After copying the files into the data directory of another server
instance, the IMPORT statement can be used to read metadata from the
SDI file(s) and inject the table definition(s) into the data
dictionary.

New statement: IMPORT
IMPORT moved from role_or_label_keyword to role_or_ident_keyword.

Thanks to Gleb Schepa who provded patches for the parser changes.

New tests:
main.import
main.import_symlink
main.import_empty_secure_file_priv