boolean isValid() {
            for (RPCTarget target : targets) {
                if ( ! target.getJRTTarget().isValid()) {
                    return false;
                }
            }
            return true;
        }
Implements a helper class holds the necessary reference and timestamp of a target. The lastUse member is updated
when a call to {@link RPCTargetPool#flushTargets(boolean)} iterates over an active target.
messagebus/src/main/java/com/yahoo/messagebus/network/rpc/RPCTargetPool.java