Tables existence check query is executed in large quantities

When Rails starts, tables existence check query is executed number of models.
In case of mysql,

    SHOW TABLES LIKE 'table1';
    SHOW TABLES LIKE 'table2';
    SHOW TABLES LIKE 'table3';
    ...
    SHOW TABLES LIKE 'table999';

Add process to get the names of all tables by one query.
I believe the lines below this one is not needed anymore because we will always have the tables inside the `@tables` hash.