resolve comments, add header, TODO, remove context in subclass

Author: sxzh93

src/executor/alter_executor.cpp

@@ -1,4 +1,3 @@
-<<<<<<< HEAD
 //===----------------------------------------------------------------------===//
 //
 //                         Peloton
@@ -11,8 +10,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-=======
->>>>>>> add rename column executor
 #include "executor/alter_executor.h"
 
 #include "catalog/catalog.h"