Summary:
`std::function` has a footprint of 32 bytes and (almost) always allocates the interrupt handler. By using an intrusively reference-counted atomic pointer the footprint is just 8 bytes, and we save further 8 bytes by eliminating `interruptHandlerSet_` (it's a `bool`, but poorly aligned). This also allows to share the handler along the continuation chain, instead of copying for every core.

In addition, the `getInterruptHandler()`/`setInterruptHandlerNoLock()` API was replaced by a single `initializeInterruptHandlerFrom()`, so we don't need to expose the internal storage details anymore.

Reviewed By: yfeldblum

Differential Revision: D22474230

fbshipit-source-id: 059828de3b89c25684465baf8e94bc1b68dac0da