Add `ByFirstMessageField` as the fallback after `ByProducerId`

[alexander-yevsyukov]

Currently we use an ID of the entity which produced an event as the default route. This is convenient for routing events produced by aggregates to projections that have the same type of the ID. But in other scenarios, especially those involving ProcessManagers, routing events by the first message field would allow to avoid multiple calls to EventRouting.unicast.

In order to make event routing more convenient, we need to have the default route passed to the EventRouting construction that initially calculates the route by the producer ID and then by the first message field.

[alexander-yevsyukov]

Addressed by #1610.