Switch to Go 1.9.4

What does this MR do?

Moves project from go 1.8.x (hard to say what is x here - we should check what was the latest 1.8.x version available when our CI image was built last time) to 1.9.4.

Why was this MR needed?

After checking the difference between 1.9 and 1.8.x I don't see many changes that would give us big benefit. However, 1.8 is now year old, 1.9 was released more than half year ago and 1.10 is in RC stage now. It's always better to use most recent version :)

Are there points in the code the reviewer needs to double check?

There are updates that are touching std packages used by us: net, net/http, sync.Mutex. After reading 1.9.0 Release Notes I don't think it'll introduce any problems, but I'm leaving this here.

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

Closes #3109 (closed), #2947 (closed)