v1 session management security work

This is a catch-all for v1 session management security work, largely inspired by the OWASP Session Management Cheat Sheet.

• HSTS
• "Secure", "HttpOnly", "SameSite" cookie attributes
• Server-side expiration
• Idle timeout
• Absolute timeout
• Cache settings to avoid leaking sensitive data
• If we get a 500 during session create (e.g., due to duplicate key, which should be impossible) sure we're not leaking info in the error message about the nature of the failure
• Review the cheat sheet and figure out what's missing
• Server side: log session lifetime
• Scope sessions to the same source IP (and maybe other request properties)
• Server side: prevent DoS by unauthenticated clients by spamming random session ids. This one deserves more explanation. I'm a little worried that if even unauthenticated requests hit CockroachDB, it's easy to DoS the control plane by just hammering Nexus with unauthenticated requests. We could mitigate this in a lot of ways. The easiest one I can think of is to severely throttle clients that have made more than a handful of requests with bogus session ids in the last few minutes. It seems like we could be very aggressive about this because well-behaving clients should rarely make requests with bogus session ids. Other ideas might be: use a separate CockroachDB cluster for this or use a separate mechanism altogether.
• Consider specifying a particular CSPRNG for token generation instead of than rand::StdRng (currently ChaCha12)

No file, test, or entry point is named; start by reviewing the OWASP Session Management Cheat Sheet and locating the v1 session-management implementation. Narrow the catch-all to one unchecked requirement, define its acceptance criteria, and verify that item before considering the work done.