Merged tables should run parallel, not sequential

[dsyzhu]

CREATE TEMP TABLE sample1 AS (SELECT * FROM sample('id1'));
CREATE TEMP TABLE sample2 AS (SELECT * FROM sample('id2'));
SELECT * FROM sample1
MERGE
SELECT * FROM sample2
OUTPUT AS samples;

sample1 and sample2 should run in parallel.

[dsyzhu]

If I remove https://github.com/yahoo/yql-plus/blob/master/yqlplus_engine/src/main/java/com/yahoo/yqlplus/engine/internal/tasks/GraphPlanner.java#L89-L121 merge will run in parallel for each data source.