Local storage of relevant users from database in WSClient at login.

Notes:
- When user registers, the list of desired/offered products is loaded from
  config file and persisted into db. When user logs in, his list of
  products is loaded from database and stored in WSC.
- Fix bug in Seller constructor
- Comment Mediator.fetch*, so launch is invalidated at this point
- TODO: Check if relevant users list in WSC is updated at
  Login Notification.