MINOR: stream-int: Add src and dst addresses to the stream-interface

MINOR: stream-int: Add src and dst addresses to the stream-interface

For now, these addresses are never set. But the idea is to be able to set, at
least first, the client source and destination addresses at the stream level
without updating the session or connection ones.

Of course, because these addresses are carried by the strream-interface, it
would be possible to set server source and destination addresses at this level
too.

Functions to fill these addresses have been added: si_get_src() and
si_get_dst(). If not already set, these functions relies on underlying
layers to fill stream-interface addresses. On the frontend side, the session
addresses are used if set, otherwise the client connection ones are used. On
the backend side, the server connection addresses are used.

And just like for sessions and conncetions, si_src() and si_dst() may be used to
get source and destination addresses or the stream-interface. And, if not set,
same mechanism as above is used.