Add extensive KDoc comments and refactor minor code components

Enhanced documentation with detailed KDoc comments for better code understanding and maintainability. Refactored parts of the MicroBankClient and actor registry classes to increase concurrency and fixed minor logical issues. Also updated the project version from 0.12.0 to 0.13.0 to mark these improvements.

actor4k-cluster/src/main/kotlin/io/github/smyrgeorge/actor4k/cluster/ClusterImpl.kt

@@ -52,7 +52,6 @@ class ClusterImpl(
     suspend fun msg(message: Envelope): Envelope.Response {
         val target = nodeOf(message.shard)
         return if (target.dc == conf.alias) {
-            println("NOOOOOOOOOOOO")
             // Shortcut in case we need to send a message to self (same node).
             grpcService.request(message)
         } else {