SSH idle timeout is too large resulting in OutOfMemoryError when clients fail to terminate the session

Currently the idle timeout for SSH sessions is currently controlled by plugin.ssh.command.timeout.idle which by default is set to 86400 seconds (1 day). This is fine when git clients use default SSH configuration which terminates SSH sessions on completion of the command but if users are using configurations such as those suggested in speed-up-git-5x-to-50x the sessions will remain in memory. Over the coarse of the day this can build up and result in an OOME. The idle timeout (which is the timeout in which no packets have been sent in either direction) should be set much lower, most likely around 30 minutes