simplify QueryStage#5

Merged

carsonwang merged 7 commits intocarsonwang:AE_1from

cloud-fan:help2

Jan 22, 2019

Commits on Jan 16, 2019

do not re-implement exchange reuse
cloud-fan
committed

Commits on Jan 17, 2019

Commits on Jan 21, 2019

address comments
cloud-fan
committed

Commits on Jan 22, 2019

improve QueryStageTrigger
cloud-fan
committed