sbws should send scanner metadata as part of every HTTP request

We can't find out which torflow instances are using a bandwidth server: https://lists.torproject.org/pipermail/tor-project/2018-December/002108.html

As far as I can tell, sbws doesn't provide any scanner info in its HTTP requests.

Here's some things we might want:

• software-name: sbws
• software-version
• scanner-nickname
• scanner-IP-address? (pro: doscover users who haven't set nickname, con: discover users)

Non-standard HTTP headers start with "X-".

Assigning to 1.0, because this is vital debugging info.