REORG/MAJOR: move session's req and resp channels back into the session

The channels were pointers to outside structs and this is not needed
anymore since the buffers have moved, but this complicates operations.
Move them back into the session so that both channels and stream interfaces
are always allocated for a session. Some places (some early sample fetch
functions) used to validate that a channel was NULL prior to dereferencing
it. Now instead we check if chn->buf is NULL and we force it to remain NULL
until the channel is initialized.
diff --git a/src/frontend.c b/src/frontend.c
index c5301af..48dabe8 100644
--- a/src/frontend.c
+++ b/src/frontend.c
@@ -189,16 +189,16 @@
 	}
 
 	if (s->fe->mode == PR_MODE_HTTP)
-		s->req->flags |= CF_READ_DONTWAIT; /* one read is usually enough */
+		s->req.flags |= CF_READ_DONTWAIT; /* one read is usually enough */
 
 	/* note: this should not happen anymore since there's always at least the switching rules */
-	if (!s->req->analysers) {
-		channel_auto_connect(s->req);  /* don't wait to establish connection */
-		channel_auto_close(s->req);    /* let the producer forward close requests */
+	if (!s->req.analysers) {
+		channel_auto_connect(&s->req);  /* don't wait to establish connection */
+		channel_auto_close(&s->req);    /* let the producer forward close requests */
 	}
 
-	s->req->rto = s->fe->timeout.client;
-	s->rep->wto = s->fe->timeout.client;
+	s->req.rto = s->fe->timeout.client;
+	s->res.wto = s->fe->timeout.client;
 
 	/* everything's OK, let's go on */
 	return 1;