pad_cookie: Move initial cookie read+save to `init()`
pad_cookie: Move initial cookie read+save to `init()`

Benefits of this change:
  * It avoids race conditions with tests that clear cookies.
  * Any attempt to get or set a value before `init()` is called will
    throw an error, ensuring the API is used properly.
  * Improved readability: It's easier to understand what the
    `pad.noCookie` check is doing.

ether/etherpad-lite,ether/etherpad-lite,ether/etherpad-lite