Move table size queries into pgcopydb catalogs (#684)

Instead of storing the table size in the source database, store it in our internal catalogs in the SQLite database. This allows better compatibility with read-only source databases.

At the moment this removes the possibility for the user to provide their own table size data in order to avoid the `pg_table_size()` query which is known to be a long running query in some platforms. A later improvement could be done to enable external table size input again.