Fix session cookie refresh when cookie options are mutated

[raashish1601]

Summary

• Track explicit session cookie mutations so cookie-only updates can emit Set-Cookie for existing sessions.
• Preserve the clean cookie state when sessions are created or loaded from the store, so unmodified sessions keep the existing non-rolling behavior.
• Add regression coverage for req.session.cookie.maxAge changes followed by touch() and save().

Fixes #1002.

Validation

• npx eslint index.js session/cookie.js session/session.js session/store.js test/session.js
• bash -lc "tr -d '\r' < test/support/gencert.sh | bash"
• npx mocha --require test/support/env --check-leaks --no-exit --reporter spec test/session.js (143 passing)