An AddUserInfo is a ReplyInfo containing the response to print on the console when an AddUserMessage has been processed.
An AddUserMessage is used to add an user with username and password to the list of users who have access to the server
A ListUserInfo is used as a ReplyInfo to format the response from a ListUserMessage.
A ListUserMessage is used to ask for the list of users who have access to the server.
A NoUserInfo is user as a ReplyInfo to handle a request that need to modify something of a user that does not exist in the 'users' map in the 'master' database.
A RemoveUserInfo is a ReplyInfo containing the response to print on the console when a RemoveUserMessage has been processed.
A RemoveUserMessage is used to remove an user from the list of users who have acces to the server.
Trait that every message that belongs to user management operations has to extend.
UserManagementMessages are used to manage the users who have access to the server.