Add hashes for each book

Check if the uploaded book is already in the library

lib/database/ro.go

@@ -45,6 +45,10 @@ func (db *roDB) IsBookActive(id string) bool {
 	return db.db.IsBookActive(id)
 }
 
+func (db *roDB) ExistsBookHash(hash []byte) bool {
+	return db.db.ExistsBookHash(hash)
+}
+
 func (db *roDB) AddUser(name string, pass string) error {
 	return errors.New("RO database")
 }