Implement db_big_sql_query() that uses cursor on PostgreSQL and limit memory usage...

By default on PostgreSQL, db_sql_query() stores the query results in
memory, it can take lots of resources when dealing with job output,
accurate or restore.  This new mode uses cursors and fetches results
100 by 100.

db_list_xxx functions doesn't store the entire query result anymore.
Commands like "list joblog" or "list files" are now stable in memory
usage. (for MySQL and PostgreSQL)