private synchronized boolean computeIsPrefetch() {
      for (Pair<Consumer<T>, ProducerContext> pair : mConsumerContextPairs) {
        if (!pair.second.isPrefetch()) {
          return false;
        }
      }
      return true;
    }
Starts next producer if it is not started yet and there is at least one Consumer waiting for
the data. If all consumers are cancelled, then this multiplexer is removed from mRequest map
to clean up.
imagepipeline/src/main/java/com/facebook/imagepipeline/producers/MultiplexProducer.java