int getRefCount() {
            int refCount = 0;
            for (RPCTarget target : targets) {
                refCount += target.getRefCount();
            }
            return refCount;
        }
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