Split SemiFuture and Future into separate types. Add BasicFuture shared between them.

Split SemiFuture and Future into separate types. Add BasicFuture shared between them.

Summary:
To avoid the risk of bugs caused by a Future being cast to a SemiFuture, and losing some of the properties in the process, this splits SemiFuture and Future into unrelated types, sharing a private superclass for code reuse.
 * Add BasicFuture in futures::detail
 * Make superclass privately inherited.
 * Unset executor when constructing SemiFuture from Future.

Reviewed By: yfeldblum

Differential Revision: D6177780

fbshipit-source-id: dea3116aeec0572bb973c2a561e17785199e86f2