Modification to futures to remove deadlock in certain use cases for getVia(executor).

Modification to futures to remove deadlock in certain use cases for getVia(executor).

Summary: If getVia was called on a future modified using via, getVia could deadlock if the original future was updated to a new executor and there was no callback chained after the call to via. In effect: f.via(executor).getVia(executor); deadlocks. This can be a problem if the code is hidden in a library and the precise semantics are unclear. This diff adds a test that exposes the problem and a fix by forcing waitVia to add a callback that will satisfy the new exector, ensuring that drive() has a callback to trigger once the future is satisfied.

Reviewed By: andriigrynenko

Differential Revision: D2906858

fb-gh-sync-id: a3105079530f15d7a7d39a9381c4078665b721a7
shipit-source-id: a3105079530f15d7a7d39a9381c4078665b721a7