Add remove-on-disconnect for Syncshells

[universalconquistador]

Some Syncshell owners would like to be able to have members who disconnect from the game be removed from the Syncshell. For example, a venue might have a very large syncshell that has no need to keep offline members around.

The server will need to keep track of when it has last seen each person, and then every minute do a job where it kicks people who are not currently connected and have not been connected for over 5 minutes.

• Syncshell remove-on-disconnect option can be set when creating a Syncshell
• Syncshell remove-on-disconnect option can be modified via the Syncshell admin window
• Backend correctly tracks the last seen time of players to a reasonable degree of accuracy
• Backend periodically identifies and removes expired users from Syncshells with the remove-on-disconnect option
• Users who have not connected since this last-seen tracking was implemented are not automatically removed from Syncshells