BUG/MEDIUM: server: do not auto insert a dynamic server in px addr_node

BUG/MEDIUM: server: do not auto insert a dynamic server in px addr_node

Until then, the servers were automatically attached on their creation
into the proxy addr_node tree via _srv_parse_init. In case of an invalid
dynamic server which is instantly freed, no detach operation was made
leaving a NULL server in the tree.

Change this mode of operation by marking the attach operation as
optional in _srv_parse_init. This operation is not conduct for a dynamic
server. The server is attached only at the end of the CLI handler when
it is marked as valid.

This must be backported up to 2.4.