Design a pluggable transport roadmap

"Disambiguation" note: This ticket is about comparing pluggable transport protocols, not about writing down the steps for deploying pluggable transports (legacy/trac#4562 (moved)).

For sponsor F deliverable 6 we promised to "help SponsorF design a roadmap for the scope of transports we expect need work." This deliverable comprises the following tasks:

• Talk to SponsorF about the prototypes of various pluggable transports they came up with and include them in the roadmap.

• Describe what security properties each of the transports aims to provide (i.e. what censorship techniques it aims to defeat).

• Figure out how far along we are at each of them and what research and development roadblocks remain.

• Prioritize transports by complexity and urgency of deployment.

• Build a plan for how to compose transports appropriately without screwing up their properties.

• For each transport, speculate about how easy and/or smart it would be to bake into Tor itself.

Optimistically assigning this ticket to Steven, because he said he might be able to lead this deliverable. This deliverable needs input from many Tor developers.