Message routers in Typed

[patriknw]

Since there is no direct replacement for routers we should document how to implement it with ordinary actors.

Also integration with Receptionist. It might turn out that we want to provide such integration as prebuilt actors.

Some inspiration can be found in the old blog posts https://akka.io/blog/2017/05/08/typed-mutable-vs-immutable and https://akka.io/blog/2017/09/28/typed-cluster

[patriknw]

Changed this from being a documentation only ticket to actually providing some basic message routers. They should be implemented as ordinary behaviors and not have the call site routing inside special actor refs. They should not be configurable. Only the basic routing strategies.

Pool router is for creating a bunch of local worker actors. No direct replacement for cluster aware pool routers.

Group routers should be implemented with the Receptionist. These will thereby be cluster aware and also dynamic.