remove references to VolcanoCost from RelSubset
This enables us to use a different RelOptCost(we are experimenting with
HiveCost).
Currently we are also extending VolcanoPlanner to override cost
functions. As discussed long term may be better to have a separate
entity for cost functions.
Merge pull request #123 from hbutani/hive

Remove references to VolcanoCost from RelSubset.