fixes issue #252

A patch to extend a cookie based session, when a user accesses sync gateway after 10% of the session expiration has elapsed, the original session ttl is added to the current system time and the session and cookie are updated and new cookie is returned to the client.

A new test has been added to the admin api
Should be renamed `sessionTimeElapsed` since it's a duration in seconds, not a percentage.
Also, there's no need to convert both of these to seconds before comparing them; Durations are numbers so they can be compared directly.