MEDIUM: server: implement 'add server' cli command

MEDIUM: server: implement 'add server' cli command

Add a new cli command 'add server'. This command is used to create a new
server at runtime attached on an existing backend. The syntax is the
following one :

$ add server <be_name>/<sv_name> [<kws>...]

This command is only available through experimental mode for the moment.

Currently, no server keywords are supported. They will be activated
individually when deemed properly functional and safe.

Another limitation is put on the backend load-balancing algorithm. The
algorithm must use consistent hashing to guarantee a minimal
reallocation of existing connections on the new server insertion.