BUG/MEDIUM: servers: Only requeue servers if they are up

BUG/MEDIUM: servers: Only requeue servers if they are up

In init_srv_requeue(), only attempt to run the tasklet if the server is
actually running, otherwise it will end up being queued a second time,
when the server is actually brought up, and that will lead to a
corrupted mt_list.
This can easily be reproduced by adding a dynamic server, as those start
disabled, and then enabling and disabling it a couple of times.
This should fix github issue #3360.

This should be backported up to 3.2.