Refactor SEQUENCE related code

[yiyun-sj]

• merge DROP statements in binder (and maybe others?)
• Use constCast instead of ku_dynamic_cast in binder (i.e. bind_ddl.cpp)
• use constPtrCast in mapper (i.e. map_ddl.cpp)
• constCast change across append_simple.cpp (and prob more in planner)
• merge logical_ddl.h with LogicalSimple which use to represent operators that only execute once