MINOR: threads/server: Add a lock to deal with insert in updates_servers list

MINOR: threads/server: Add a lock to deal with insert in updates_servers list

This list is used to save changes on the servers state. So when serveral threads
are used, it must be locked. The changes are then applied in the sync-point. To
do so, servers_update_status has be moved in the sync-point. So this is useless
to lock it at this step because the sync-point is a protected area by iteself.