(Wangle) Make via behave more like gate

(Wangle) Make via behave more like gate

Summary:
Could the problem be that via continues the existing chain of futures,
whereas we actually want to start a new chain?

Is there any particular reason this wasn't implemented like this originally?

Test Plan:
Ran all the unit tests. I hope to try to reproduce the thread issue and
see if this improves things.

Reviewed By: davejwatson@fb.com

Subscribers: trunkagent, net-systems@, exa, njormrod, fugalh

FB internal diff: D1500225

Tasks: 4920689