MINOR: server: add the "cc" keyword to set the TCP congestion controller

MINOR: server: add the "cc" keyword to set the TCP congestion controller

It is possible on at least Linux and FreeBSD to set the congestion control
algorithm to be used with outgoing connections, among the list of supported
and permitted ones. Let's expose this setting with "cc". Unknown or
forbidden algorithms will be ignored and the default one will continue to
be used.