CBO enhancements and fixes after calibration tests

This issue is created to fix any CBO issues that arise after the scan and join calibration tests are run.

The following issues were fixed:

• Cost based optimization of scans. Prior to this, only queries with (inner) joins were considered for cost based optimization.
• Fixed bug in printing of join costs in the explain plan. 
• Separated out exchange costs from join operator costs. Previously exchange costs were part of the join operator to which the exchange was sending data. This makes it easier for debugging and is more intuitive.
• Added explicit overflow component to the Cost vector. Previously the overflow was embedded in the docsProcessed component. This makes it easier for debugging.
• Enhanced the costing formula for index nested loops join based on the calibration test results.
• Added more details to the logging routines for dumping of join nodes during join enumeration.