BUG/MEDIUM: server: do not forget to generate the dynamic servers ids

BUG/MEDIUM: server: do not forget to generate the dynamic servers ids

If no id is specified by the user for a dynamic server, it is necessary
to generate a new one. This operation is now done at the end of 'add
server' CLI handler. The server is then inserted into the proxy ids
tree.

Without this, several features may be broken for dynamic servers. Among
them, there is the "first" lb algorithm, the persistence using
stick-tables or the uniqueness internal check of srv_parse_id.

This must be backported up to 2.4.