Synced access to server connections.
Merge remote-tracking branch 'psimeonov/synchronized-map'
Container where we keep connections is of interface type Map
Fixed nasty bug with updating map key to contain the userName istead of client ip
Added new type SystemMessage that will be sent to inform the users who's online
Added function returning comma separated list of online users
Use JList to display online users in ClientView
MessageHandler can now update the list of online users
Server now sends a SystemMessage with all online users upon each client connection
Added function to send SystemMessages to all users to update their online lists
Clear user list and fill it again on every SystemMessage that contains new user list info
Merge branch 'fixes'