Keep consistent data stream semantic for streaming query

[yl-lisen]

Describe what enhancement you'd like to have Currently, the below both queries have inconsistent data stream semantic:

-- output is append-only
SELECT * FROM left_stream JOIN right_stream using(id);
--- output is changelog
SELECT * FROM left_kv JOIN right_kv using(id);

Expected behavior: by default is append-only

--- output is append-only
SELECT * FROM left_stream JOIN right_stream using(id);
--- output is append-only
SELECT * FROM left_kv JOIN right_kv using(id); 
--- emit changelog explicitly
SELECT * FROM left_kv JOIN right_kv using(id) EMIT CHANGELOG;