Allow for setting wlm_query_slot_count for Redshift imports

[smugryan]

Redshift/postgresql has massive amounts of knobs to turn for optimizing query performance and one recommended by AWS is increasing wlm_query_slot_count in a session before running a query that needs extra CPU and memory.

Docs: http://docs.aws.amazon.com/redshift/latest/dg/cm-c-defining-query-queues.html

Would be awesome to allow users to set this in the redshift config. We saw massive (10x faster vacuums) improvements when increasing ours to 45, but we need to do that manually for now.

[alexanderdean]

Nice idea!