FD_SOCK server socket is never closed after network interruption

JGroups FD_SOCK protocol did not set the <systemitem>TCP_KEEPALIVE</systemitem> option on the server side of the socket. After a network interruption during which the client side had broken the connection, the server side sockets remained open until the JGroups channel was shut down. The FD_SOCK protocol now sets the <systemitem>TCP_KEEPALIVE</systemitem> option on the server side of the socket. Now even if the client side of the socket is closed during a network interruption, the server side socket is closed by the <systemitem>TCP_KEEPALIVE</systemitem> timeout.