WL#4545: Add TABLESPACE_NAME field to I_S.TABLES

After I_S.FALCON_TABLES was removed, the only way to determine tablespace that
table uses is `SHOW CREATE TABLE`. There should be a way to retrieve this
information via INFORMATION_SCHEMA.TABLES. Required at least for backup (see
WL#4264).

Add a new field to INFORMATION_SCHEMA.TABLES with the following definition:
`TABLESPACE_NAME` varchar(64) DEFAULT NULL

TABLESPACE_NAME field should be added before `TABLE_COMMENT` field.

When querying I_S.TABLES, return tablespace name if table has this option. If a
table is not bound to a specific tablespace, return NULL.

In sql_show.cc:
- add `TABLESPACE_NAME` to tables_fields_info[] array;
- modify get_schema_tables_record(), so it stores tablespace name to that field;
- fix indexes, so `TABLE_COMMENT` is stored properly;
- we do not need to open a table to determine tablespace name - we can get it
from .frm.