code: public Retryer<R> withBlockStrategy(BlockStrategy blockStrategy) {
          checkState(this.blockStrategy == null, "a block strategy has already been set %s", this.blockStrategy);
          this.blockStrategy = checkNotNull(blockStrategy, "BlockStrategy cannot be null");
          return this;
      }
docs: Sets the block strategy used to decide how to block between retry attempts.
  The default strategy is to use {@link Thread#sleep(long)}
  
  @param blockStrategy
  @return