Enumerator & mapper improvements

[andyfengHKU]

• Rename enumerator to planner except for join order enumerator
• Keep schema as planner local concept. Avoid giving schema to mapper. A possible solution is, instead of giving actual factorzation group position to logical operator. We give a logical position and mapper can map from logical position to a physical position.
• Split mapper to different cpp file per operator.

[andyfengHKU]

For the second task, we choose a different approach. Now we are giving schema to operator so dependency becomes

physical operator -[depends on]-> schema -[depends on]-> logical operator. We can move logical operator freely as long as we recompute schema after moving. See PR #1119 and #1121