Summary:
Callback destruction can involve user logic.  In practice, we
found quite a bunch of patterns like:

```
executor->add([x = someGuardObjectThatTriggersWorkInDtor] {
  ...
});
```

So ensure that dtor is

* Executed under `RequestContextScopeGuard`,
* Protected from leaking unhandled exceptions.

Reviewed By: ot, luciang

Differential Revision: D26744394

fbshipit-source-id: 5fcca0287a98de919c3649a9613e6d54cc1a1ba3