Roadmap for NEW_TOKEN utilization

[gretchenfrage]

Tracking issue for anyone interested in my attempt to implement NEW_TOKEN utilization. Thanks so much to Ralith and djc for the effort spent workshopping this with me so far!

Already done:

• Minor style changes #2081
• Adjust terminology regarding tokens #2086
• proto: Connection side enum #2084
• proto: Refactor TokenDecodeError #2085
• Make SystemTime mockable via object #2089

Anticipated future dependency path:

• 1. Factor out IncomingToken #2094
• 2. Token tweaks #2098 (depends on 1.)
• 3. proto: Pass SocketAddr by value #2107 (depends on 2.)
• 4. proto: Pass ConnectionId by value internally #2109 (depends on 2.)
• 5. proto: Token encode refactors #2110 (depends on 3, will have minor merge conflicts with 4.)
• 7. Utilize NEW_TOKEN frames #1912 (depends on 5.) (note: implementation of BloomTokenLog and InMemTokenStore can be removed from 1912 and shunted to follow-up PRs)
• 8. Add BloomTokenLog #2136 (depends on 7.)
• 9. Add TokenMemoryCache #2137 (depends on 7, thus can be created simultaneously to 8.)

Possible future followup work:

• Allow more manual usage of tokens on client side
• Allow more manual usage of tokens on server side
  • Allow encrypted application data to be placed in token