Add

• #56 - Add: keep stats about the openttd-version used for listing (@TrueBrain)

Fix

• #55 - Fix: if a server leaves while a client is connecting, abort the attempt (@TrueBrain)
• #54 - Fix: exception if a server leaves during registration (@TrueBrain)
• #53 - Fix: clean up verify tokens once we are done with them (@TrueBrain)
• #52 - Fix: one client could attempt multiple simultaneous connections to the same server (@TrueBrain)