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/proxy.c b/src/proxy.c
index 405c4c4..a8cb443 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -973,8 +973,8 @@
 		s->txn.req.flags |= HTTP_MSGF_WAIT_CONN;
 
 	if (be->options2 & PR_O2_NODELAY) {
-		s->req->flags |= CF_NEVER_WAIT;
-		s->rep->flags |= CF_NEVER_WAIT;
+		s->req.flags |= CF_NEVER_WAIT;
+		s->res.flags |= CF_NEVER_WAIT;
 	}
 
 	/* We want to enable the backend-specific analysers except those which
@@ -982,7 +982,7 @@
 	 * be more reliable to store the list of analysers that have been run,
 	 * but what we do here is OK for now.
 	 */
-	s->req->analysers |= be->be_req_ana & ~(s->listener->analysers);
+	s->req.analysers |= be->be_req_ana & ~(s->listener->analysers);
 
 	return 1;
 }