ORPort directory requests cropped

Great catch from starlight that if descriptors downloaded via an ORPort
exceeding a certain size get cropped...

  https://trac.torproject.org/projects/tor/ticket/28961

There were a couple issues here...

  * Our socket handling was pretty screwed up. Calling socket's recv() provides
    data available at the time, so if we haven't received the a full cell we
    should pull more.

  * Descriptors can be composed of multiple RELAY cells (the descriptor was
    cropped because we stopped reading after the first). Directory requests
    now keep reading cells we receive the END signal.