Separate cookie maxAge option from external store ttl

[ejose19]

This PR allows to individually control cookie maxAge and external store TTL, for the cases where an unified control is not desired (like using with koa-redis).

Usage:

• maxAge remains the same
• ttl default is maxAge, unless maxAge is set to 'session' and ttl defaults to ONE_DAY
  allowed values for ttl are: 'permanent' (passes undefined ttl to external store) or any integer value

Previously using maxAge = 'session' with koa-redis would give an out of range error (since koa-redis is trying to insert word 'session' under ttl, with this implementation it will pass an integer value to ttl or if set as 'permanent', undefined and koa-redis will store without any ttl)

[coveralls]

Coverage remained the same at 100.0% when pulling 2f6926f on ejose19:master into f765595 on koajs:master.