async initialization of Client

[dawuud]

Client, among other things, follow a pattern like this:

• their __init__ has a bunch of non-trivial code, including creating other objects and possibly doing async things
• if other things need to know "when they're ready" there are tricks like when_ready methods.

Instead this code should be written like this:

• a create_thing factory-method (that is async) is created
• all non-trivial objects are instantiated outside and passed it
• IService and startService should be used solely for startup notification