Summary:
[Folly] Lock utilities: `upgrade_lock`, `transition_to_shared_lock`, `transition_to_upgrade_lock`, `transition_to_unique_lock`, `try_transition_to_unique_lock`.

Note: `std::shared_mutex` does not have an upgrade state or any atomic transitions; there is no `std::upgrade_lock`; and `std::shared_lock` does not have a constructor from `std::unique_lock`.

Reviewed By: simpkins, nbronson

Differential Revision: D15532568

fbshipit-source-id: d32f387b26b57539857922c3090dc75ffcaaa538