MINOR: tcp_samples: Add samples to get src/dst info of the backend connection

MINOR: tcp_samples: Add samples to get src/dst info of the backend connection

This patch adds 4 new sample fetches to get the source and the destination
info (ip address and port) of the backend connection :

 * bc_dst      : Returns the destination address of the backend connection
 * bc_dst_port : Returns the destination port of the backend connection
 * bc_src      : Returns the source address of the backend connection
 * bc_src_port : Returns the source port of the backend connection

The configuration manual was updated accordingly.