Event router plugin API

[krasserm]

At the moment, routing of written event to their registered event-sourced destinations (actors, view and processors) is based on hard-coded logic. Applications should be able to plugin their custom routing logic. This could be used to implement #46.

[krasserm]

Two cases must be distinguished here:

• With persistent routing decisions, the destination aggregateIds are persisted with the events, both in the raw event log and in the index. A persistent router can be easily implemented on top of the existing persist and persistOnEvent API that take a customDestinationAggregateIds parameter. Here, the emitter make a routing decision, and persists that decision by setting customDestinationAggregateIds. Now special router plugin API is be needed in this case.
• With transient routing decisions, the destinations are not persisted with events. By replaying events from the raw event log, routing decisions can also be replayed. This is to some extend comparable to handling decisions made by event handlers i.e. whether an event should be handled or not. This routing approach however is not compatible with replaying events from an aggregateId-based index as the index hasn't recorded custom routing decisions, so only a subset of the originally routed events would actually be replayed to a given destination which is a causality violation.

[krasserm]

Won't implement for reasons explained in previous comment.