Make CoreAndCallbackReference move-ctor faster, release memory eagerly/sooner
Make CoreAndCallbackReference move-ctor faster, release memory eagerly/sooner

Summary:
Avoid swap() in move-ctor:
  - slightly faster
  - frees resources eagerly/sooner vs. later

Side benefit: added `noexcept` to a few appropriate methods.

Reviewed By: yfeldblum

Differential Revision: D7723078

fbshipit-source-id: f2d08b5ce094c42cb114deda7a6841052d1b785a

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly