[MEDIUM] enable inter-stream_interface wakeup calls

By letting the producer tell the consumer there is data to check,
and the consumer tell the producer there is some space left again,
we can cut in half the number of session wakeups.

This is also an important starting point for future splicing support.
diff --git a/src/client.c b/src/client.c
index 4e8004e..5885f06 100644
--- a/src/client.c
+++ b/src/client.c
@@ -182,6 +182,8 @@
 		s->si[0].owner = t;
 		s->si[0].shutr = stream_sock_shutr;
 		s->si[0].shutw = stream_sock_shutw;
+		s->si[0].chk_rcv = stream_sock_chk_rcv;
+		s->si[0].chk_snd = stream_sock_chk_snd;
 		s->si[0].fd = cfd;
 		s->si[0].flags = SI_FL_NONE;
 		s->si[0].exp = TICK_ETERNITY;
@@ -192,6 +194,8 @@
 		s->si[1].owner = t;
 		s->si[1].shutr = stream_sock_shutr;
 		s->si[1].shutw = stream_sock_shutw;
+		s->si[1].chk_rcv = stream_sock_chk_rcv;
+		s->si[1].chk_snd = stream_sock_chk_snd;
 		s->si[1].exp = TICK_ETERNITY;
 		s->si[1].fd = -1; /* just to help with debugging */
 		s->si[1].flags = SI_FL_NONE;