Scope secondary authentication to login sessions

[rizen]

Summary

• bind pending and completed secondary authentication to the current Wing session
• prevent impersonation or an earlier login from authorizing a later session for the same user
• return an actionable error if verification is requested without a live session

TDD evidence

• Red: author.t/secondary_auth_session.t failed because verification for session-1 was accepted by session-2
• Green: prove -lv author.t/secondary_auth_session.t author.t/session_absolute_expiry.t passes 16 assertions

Fixes plainblack/TGC#1271