Modify redshift adapter to support spectrum

[ragi256]

This PR is for support Redshift Spectrum. The modification is only redshift_adapter.rb

• fetch_table_names method change to use table from pg_tables to svv_tables
  • the table has also external table information
• RedshiftAdapter has instance variables for BASE TABLE, EXTERNAL TABLE, VIEW
  • to check whether a table is external table or not in fetch_rows / fetch_count
• If importing table is external, about raw dataset
  • The price of spectrum depends on the amount of scanning data, but it is difficult to limit the volume in fetch_rows / fetch_count.
  • fetch_rows : need to use partition column to reduce scan.
  • fetch_count : spectrum tables do not have meta data about record size.

[ragi256]

This method depends on execution order because needs @external_table_names to check spectrum tables or not.
RedshiftAdapter is only used import tables ( synchronize_data_sources.rb).