Make ProcessManagers maintain their state

[andreyleskov]

Now ProcessManagers will ask their State actor to load the state for them.
Initially, it looked like a good idea. But now I can't see any benefits except simpler code base.
This decision creates a big limitation for process manager state - it should be serializable.
Let PM inherit from aggregate actor to load the state itself and provide full flexibility for state aggregate, including DI and domain services usage.