Deliverable: Vanguard design for protection against guard discovery attacks is implemented in Arti.

We will implement the protection against guard discovery attacks with a design called Vanguards. To do this we must:

• Create separate instances of GuardMgr (or GuardSample) to track chosen vanguard sets.
• Select vanguards according to the specified rules; update them when they fail; rotate as appropriate.
• Persist vanguards to our state files.
• Implement circmgr settings to use vanguards as appropriate depending on circuit purposes, and build circuits with the right lengths.
• Investigate relay bandwidth reporting for our metrics services during flooding attacks.
• Improve handling of protocol-invalid cells in Arti so that these cases cannot be used for side-channels

Milestone 2.1.0 for ZC