message: Log a debug message if a stage is congested
diff: #include "caf/downstream_msg.hpp"
  #include "caf/inbound_path.hpp"
  +#include "caf/logger.hpp"
  namespace caf {
  namespace policy {
  @@ -55,7 +56,10 @@ auto downstream_messages::id_of(mailbox_element& x) noexcept -> key_type {
  }
  bool downstream_messages::enabled(const nested_queue_type& q) noexcept {
  - return !q.policy().handler->mgr->congested();
  + auto congested = q.policy().handler->mgr->congested();
  + CAF_LOG_DEBUG_IF(congested, "path is congested:"
  + << CAF_ARG2( "slot", q.policy().handler->slots.receiver));
  + return !congested;
  }
  } // namespace policy
  