Resolve race condition on Mediator

[ainoya]

It was detected race condition about execution stage by go test -race race condition detector. I resolved this racing,

• Mediator that mediate execution result between stages was single struct. Because it was shared with all stages and written execution results by them at the same time, it causes race condition.
• I divide the mediator each stages, make a mediator to have always single result of single stage.

By this changes, multiple mediators communicate with input and output channel of a stage. So next(future PR) I'll try to process multiple mediators,

• Bind the multiple mediators to summary result.
• Failover system by reading mediators. #26

[ainoya]

Please check it out, @takahi-i

[takahi-i]

Thanks @ainoya! I will merge this pr tough i have not look into the details (I will check the details tomorrow).