Ensure maxAge type has milliseconds as the unit

[chuanqisun]

• This PR is meant to replace Fix: maxAge description in readme #227
• This PR is a two step plan to fix the maxAge unit issue. See follow-up step in Wrong type description for maxAge #244

Checklist

• run npm run test and npm run benchmark
  • run npm run test
  • run npm run benchmark ⚠️ unable to run due to TypeError: redisStoreFactory is not a function
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[gurgunday]

lgtm

[gurgunday]

Putting for reference:

session/lib/cookie.js

Lines 41 to 45 in 8554dca

	set maxAge (ms) {
	this.expires = new Date(Date.now() + ms)
	// we force the same originalMaxAge to match old behavior
	this.originalMaxAge = ms
	}

session/lib/session.js

Lines 59 to 63 in 8554dca

	touch () {
	if (this.cookie.originalMaxAge) {
	this.cookie.expires = new Date(Date.now() + this.cookie.originalMaxAge)
	}
	}