Make sure HTTP storage respects privacy settings

[itamarst]

On client side:

• Tor is not used by client when requested (#4029)
• I2P is not used by client when requested (no ticket yet)

On server side:

• The server will expose its IP regardless of privacy settings. (I've spent less time learning about this so this may be nonsensical...)

[itamarst]

1. Tor is done and works.
2. If I2P is set ... it will blow up. So it will fulfill the requirements in this ticket.
3. For servers, all the listening code is currently tied to Foolscap, so it's the same code path and shouldn't be an issue. When eventually Foolscap is ripped out, the code will need to be ported to be HTTP only, but that is not an immediate concern.