Refactor handlers

[xepozz]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	✔️

Drop support pre-configured handlers, make it compatible with validator/hydrator handlers way

The old way:

1. Configure config/params
2. Add handler name => handler definition
3. Add handler name to the message getHandlerName method

The new way:

1. Create a class that implements the new MessageHandlerInterface
2. Add the class name to the message getHandler method

The new way is similar with validator's or hydrator's way.