Simplified/enhanced explain plans with a total of four explain levels.

There are now 4 explain levels summarized as follows:
- Level 0: MINIMAL
  Non-fragmented parallel plan only showing plan nodes with minimal attributes
- Level 1: STANDARD
  Non-fragmented parallel plan with some details in plan nodes
- Level 2: EXTENDED
  Non-fragmented parallel plan with full details in plan nodes including
  the table/column stats, row size, #hosts, cardinality,
  and estimated per-host memory requirement
- Level 3: VERBOSE
  Fragmented parallel plan with full details (like level 2)

This patch also includes several bugfixes related to plan costing and/or
testing of explain plans.

Change-Id: I622310f01d1b3d53ea1031adaf3b3ffdd94eba30
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1211
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins