Provide a way for YP Dirs to check streams

directory server GUID checks

directory server does GET /GUID-asldjfasldfjalsdkfjasldkfj HTTP/1.0 and either gets a 404 if it's wrong, or a 200 if it's correct.

As suggested already in early drafts of the YP Protocol, there should be a proper way to check if a stream is still reachable and running (and even maybe fetch updated metadata while at it).

This could be solved by implementing HTTP 1.1 HEAD and setting some headers, or as the TODO suggest, making a dedicated endpoint for it.