Allow usage of message handler classes as message handler names

[viktorprogger]

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

This PR allows usage of message handler class names as handler name in a message. This approach allows to skip creation of handler name to callable mapping for the Worker configuration. It's handy for situations when a single application plays as both producer and consumer. For situations when an application is either a producer or a consumer, it's still possible to use mapping, as it was before. Handler class name usage is not a requirement.