WithMessageHandler overload not helpful

[AnthonySteele]

Having both of these overloads to register a handler makes some proposed new features harder:

        WithMessageHandler<T>(IHandlerAsync<T> handler) where T : Message;

        WithMessageHandler<T>(IHandlerResolver handlerResolver) where T : Message;

The first can't be used with any system that passes transient data to the handler instance constructor.

A new handler instance each time seems safer in general, otherwise it requires handler authors coding for re-entrancy, and all dependencies being long-lived, which is all not obvious.

Describe the solution you'd like

Remove the overload that takes a handler instance.
It is used in tests, but a dummy IHandlerResolver that only returns a supplied instance could be used instead there.

Additional context

Considered as useful for some kind of message context accessor injected into handler constructor.

[adammorr]

I believe WithMessageHandler<T>(...) isn't a thing anymore with the changes @martincostello made to the fluent API

[martincostello]

Yeah, this is an artifact of the design we want to move away from. The new API internally just uses the variant that uses the resolver.

https://github.com/justeat/JustSaying/blob/ef0e4a6b16cdef4388a948650866e79c351da48a/JustSaying/Fluent/TopicSubscriptionBuilder%601.cs#L104

This should get tidied up as part of #471.