MINOR: tcp: Return TCP statistics like RTT and RTT variance

MINOR: tcp: Return TCP statistics like RTT and RTT variance

This patch adds 4 new sample fetches which returns the RTT of the
established connexion and the RTT variance. The established connection
can be between the client and HAProxy, and between HAProxy and the
server. This is very useful for statistics. A great use case is the
estimation of the TCP connection time of the client. Note that the
RTT of the server side is not so interesting because we already have
the connect() time.