MINOR: session: start to reintroduce struct session

There is now a pointer to the session in the stream, which is NULL
for now. The session pool is created as well. Some parts will move
from the stream to the session now.
diff --git a/src/stream.c b/src/stream.c
index 3bc337e..aaea9c0 100644
--- a/src/stream.c
+++ b/src/stream.c
@@ -116,6 +116,7 @@
 
 	memset(s->stkctr, 0, sizeof(s->stkctr));
 
+	s->sess = NULL;
 	s->listener = l;
 	s->fe  = p;