Save cover color so book does not change each run

- BookInfo loses the (random, generated) cover color property
- Code for generating random cover colors moves to Book
- BookServer is responsible to create one for a book that doesn't
  have it already
- Places that used to set it no longer need to
What an we call this that will be more accurate than "random"? I don't know how to say it accurately myself. It's something like an assignment to a small number of sequentially rotating choices :-)