Seek support on read operations #86

I'm implementing a service using reactphp/filesystem that allows for data transfer supporting resume operations. In order to accomplish this, I need to be able to seek() to a particular offset in the file.

I've found that the Adapter::read method contains a reference to the offset, passed in as the third parameter.

    public function read($fileDescriptor, $length, $offset)
    {
        return $this->fileDescriptors[$fileDescriptor]->rpc(Factory::rpc('read', [
            'length' => $length,
            'offset' => $offset,
        ]))->then(function ($payload) {
            return \React\Promise\resolve(base64_decode($payload['chunk']));
        });
    }

However, this $offset seems to only be used internally by the ReadableStreamTrait class, by keeping a reference to the current readCursor and incrementing this by chunkSize each read operation.

Is there any way to set the reacCursor position manually or do I need to look into using the Adapter / filesystem class directly? Would it be worth adding a new method to the ReadableStreamTrait to set the readCursor directly in order to set the offset?