INT-267: Rework `RoutingSlip` -> `Map<List<String>, Integer>`

Since `RoutingSlip` POJO isn't scalable in the distributed multi-language environment,
it would be better to use some Java generic type for this `ROUTING_SLIP` header.

The `Collections.singletonMap(Collections.unmodifiableList(routingSlipPath), 0)` is the best candidate to be convertible to other systems
and allow to have thread-safety.

The `ROUTING_SLIP` header is recalculated now on each `nextPath`