void subRef() {
        if (ref.decrementAndGet() == 0) {
            target.close();
        }
    }
<p>This method is used for explicit reference counting targets to allow
reusing open connections. When the reference count reaches 0, the
connection is closed.</p>

@see #addRef()
messagebus/src/main/java/com/yahoo/messagebus/network/rpc/RPCTarget.java