Synchronized: disable operator= when the type isn't copy assignable

Synchronized: disable operator= when the type isn't copy assignable

Summary: The value of std::is_copy_assignable<folly::Synchronized<T>>::value is incorrect when T isn't copy assignable. As a result, it isn't possible to use SFINAE to properly select a function when the base type is a folly::Synchronized. This diff selectively deletes the copy constructor and copy assignment operator when the underlying type T isn't copyable. This is most useful in the case of folly::Synchronized<std::unique_ptr<...>>

Reviewed By: yfeldblum

Differential Revision: D4203081

fbshipit-source-id: 1e811f9e52db26c23b1c6f1907bac9e2854ddb9d