Extend CookieStore functionality to allow access to underlying store

[pfernie]

As a continuation of #14 , this issue is to discuss making the underlying cookie store available to users of Client.

Desired functionality:

• Allow builders of a Client to provide a pre-populated CookieStore
• Allow users of a Client to access and potentially modify the CookieStore between requests
• Allow users of a Client to get the current state of the CookieStore to allow persisting session state.
• Allow alternative implementations of a CookieStore, such as a read-only store.

[seanmonstar]

Done in #1203