Refactor Context in Cascades:
use two context in cascades framework.
JobContext is used in each job, contains such attributes:
- reference to PlannerContext
- current cost upper bound
- current required physical properties
PlannerContext is used to hold global info for query planner, contains such attributes:
- reference to Memo
- reference to connectContext
- reference to ruleset could be used for plan
- job pool to maintain unexecuted jobs
- job scheduler to schedule unexecuted jobs
- current job context for next job to be executed
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