Nomenclature in Synchronized - prefer read to shared

Nomenclature in Synchronized - prefer read to shared

Summary:
[Folly] Nomenclature in `Synchronized` - prefer `read` to `shared`.

`folly::Synchronized` is a higher-level abstraction, so it will use higher-level nomenclature than will lower-level tools like `std::mutex` and `folly::LockTraits`. `shared` describes the lock state, and is used in the C++ libraries. `read` describes the class of operations that calling code is permitted to perform.

Reviewed By: simpkins

Differential Revision: D3840060

fbshipit-source-id: 4b23eaa391cb59d1eca2bfacf72db89d3c7c591e