OkHttpClient should be treated as immutable once shared

...given that requirement, it's safe for many threads to call the
open() method concurrently to open their own connections.

(Manually cherry picked from b8f3a029d3c59f037fd42eb2aed7ff926d62db8b, 8a0b263b7a8a8ceae0bf72f8b463ee78f7963b80, and 6b17f9edf338173c6491cc30feeb6e4ff539be9d)