Skip generating sequence operator if it only has one child operator

The planner generates a sequence operator in many cases, in particular it does not check how many child operators are there before generating the new sequence operator. We could do an optimization at runtime to avoid generate a sequence operator if it only has a single child operator. This is somewhat similar to the parallel operator, where the planner builds one in, but at execution time we only generate a parallel operator if maxparallelism is not 1.