Allow users to place holds on an unavailable book

[albendz]

Summary

Allow users to place holds on a book

Details

A user can create a hold on a book when they want to borrow a book that does not have available copies. This will create a hold entity in the holds table.

This operations is through a REST API: /book/{isbn}/hold with member and copy (optionally) in payload as with #18

This will place an asynchronous message on a kafka topic to place a hold. The kafka consumer will read the hold request and create the hold entity in the database.

If a member, book, or copy does not exist, return 404.

A user can only have 20 active holds. If a user has 20 hold already, return a 400.