Make `create_client()` return a singleton client

We've discussed this multiple times, time to do it.

`create_client()` and `create_async_client()` should create a client once upon the first call (one sync singleton and one async singleton respectively) and follow on calls should just return that first thing.

* Add `detached=False` argument to force creating a new client when flipped to `True`
* Obviously singletons should be partitioned by connection arguments
* Use `weakref.WeakValueDictionaty[ConnectionSpec, Client]`