(wangle) Interrupts (and therefore, cancellation)

(wangle) Interrupts (and therefore, cancellation)

Summary:
Modeled very closely after Finagle's interrupts. Compare with https://github.com/twitter/util/blob/master/util-core/src/main/scala/com/twitter/util/Promise.scala if you like.
The basic idea is the promise holder can register an interrupt handler, and then interrupts will call that handler. A typical handler would fulfil the promise with an exception (or special value) indicating that it was interrupted (if it was interrupted in time).
Raising an interrupt does not prevent setting a value or callbacks executing or any of that - it is only advisory to the promise holder.

Test Plan: I wrote some unit tests.

Reviewed By: davejwatson@fb.com

Subscribers: folly-diffs@, net-systems@, fugalh, exa, hannesr, njormrod

FB internal diff: D1620805

Tasks: 4618297