Unify default routing conventions for signal messages

[alexander-yevsyukov]

We need to unify the way we route signals in the default scenario.

Current defaults for commands

It's done by the first message field of a command message.

Current defaults for routing events

Currently, these defaults for routing events are set differently depending on the class of a repository.

• EventDispatchingRepository and AggregateRepository create event routing with the default via EventRouting.withDefaultByProducerId().
• ProjectionRepository descends from EventDispatchingRepository and does not alter the defaults. So it's withDefaultByProducerId() too.
• ProcessManagerRepository — which descends from EventDispatchingRepository too — overrides the defaults set by the parent class to EventRoute.byFirstMessageField().

So, now we mostly route by the producer ID stored in EventContext. This is a source of confusion by itself, because it requires remembering the rules by the kind of an entity.

Suggested unification

• Always have the default routing by the first message field for all kinds of signals.
• Use EventRoute.byProducerId() for EventDispatchingRepository if the type of the first message field does not match the ID of the entities.