Obsolete then schedule for removal all sync client API that layer over transports and storage

[marcschier]

We have tremendous code duplication on the client side where every api has a sync and async variant. Since async api is most efficient and performant for network and storage bound calls, we want to do a 3 step process:

• Remove all sync api in client, session and subscription etc., especially interfaces in release n
• Remove all tests for synchronous calls
• provide sync api on the concrete implementations that layer over async (via GetResult().GetAwaiter()) that are marked [Obsolete] to support incremental porting to async in release n, mark the sync api as code coverage excluded.
• Remove the wrappers in release n + x (x = tbd)

[marcschier]

Fixed with #3174