MINOR: server: add SRV_F_DELETED flag

MINOR: server: add SRV_F_DELETED flag

Set the SRV_F_DELETED flag when server is removed from the cli.

When removing a server from the cli (in cli_parse_delete_server()),
we update the "visible" server list so that the removed server is no
longer part of the list.

However, despite the server being removed from "visible" server list,
one could still access the server data from a valid ptr (ie: srv_take())

Deleted flag helps detecting when a server is in transient removal
state: that is, removed from the list, thus not visible but not yet
purged from memory.