Bakery 3.x and pluggable account management

Problem
Bakery is hard to customize for specific use cases:

• 7.x-2.x does not do user field sharing (see #1107692: Port bakery 6.x-2.x to 7.x-2.x → for details)
• Registration and login redirect can break too easily
• Bakery's methods for uniquely identifying user across sites is out-dated and should now use UUIDs
• Bakery development is difficult because of cookie metaphors and use
• Hard to test

Proposed solution

Simplify Bakery down to what it does best: creating and validating a cookie and authentication

Employ a hook system to allow site-specific customization of user-related process flows and data management (related #556666: Sync hooks: Enable sharing of arbitrary data → )