BUG/MINOR: server: do not keep an invalid dynamic server in px ids tree

BUG/MINOR: server: do not keep an invalid dynamic server in px ids tree

A bug is present when trying to create a dynamic server with a fixed id.
If the server is detected invalid due to a later parsing arguments
error, the server is not removed from the proxy used ids tree before
being freed.

Change the mode of operation of 'id' keyword parsing handler. The
insertion in the backend tree is removed from the handler and is not
taken in charge by parse_server for configuration parsing. For the
dynamic servers, the insertion is called at the end of the 'add server'
CLI handler when the server has been validated.

This must be backported up to 2.4.