Merge minimal Tor integration for future zebra-client work

[teor2345]

Motivation

We want to merge a minimal Tor integration into zebra-network, so that zebra-client can use Tor to submit user transactions. Tor support will give zebra-client basic network privacy.

This change also provides a foundation for exploring designs that provide additional network privacy in future.

Analysis

We don't want to commit to a more complex design yet, because:

• basic tor support is a definite improvement
• we're focused on other work right now
• we want to look at the tradeoffs of the more complex alternative designs
• changing network privacy implementations splits the anonymity set, so we want to explore alternatives in the design phase, before we merge a more complex design into the main branch

Tasks

• Copy the basic tor support sections from the Stolon design and merge them as a design RFC
• Add a draft RFCs folder
• Merge the Stolon design as a draft:
  • note that it's a draft,
  • add a link to the merged minimal design RFC, and
  • add a link to the Stolon implementation PR
• Copy the basic tor support code from the Stolon implementation, and merge it into zebra-network
  • add a note saying that it should be used to submit transactions (rather than syncing the whole chain)
• Close the Stolon implementation PR
  • note that it's a draft
  • add a link to the merged minimal design RFC
  • add a link to the draft Stolon RFC
• Make a Zcash forum post saying that zebra-client aims to use tor to submit user transactions
  • Note: we've already talked about Tor and zebra-client on the arti ZOMG post at https://forum.zcashcommunity.com/t/arti-a-pure-rust-tor-implementation-for-zcash-and-beyond/38776/14
  • say that we have a zebra-client design, but we're focused on validation right now
  • add a link to the merged minimal design RFC
  • add a link to the Stolon implementation PR

Follow-Up Work

Implement zebra-client with tor support

Explore more complex alternative network privacy designs