BUG/MEDIUM: stream-int: don't immediately enable reading when the buffer was reportedly full

There is a long-time issue which affects some applets, at least the stats
applet. If a large stats page is read over a slow link, regularly the
channel's buffer contains too many response data to allow another round
of ci_putblk() to copy a new message. In this case the applet calls
si_applet_cant_put() to mention that it failed to emit data into the
channel's buffer, and wants to be called only once some room is made.

The problem is that stream_int_update(), which is called from
process_stream(), will clear this flag whenever it sees there's some
spare room in the channel's buffer. It causes the applet to be woken
again immediately. This is very visible when reading a large stats
page over a slow link, because in this case haproxy will run at 100%
CPU and strace shows mostly epoll_wait(0). It is very likely that some
other applets like CLI, Lua, peers or SPOE have also been affected but
that the effect were less noticeable because it was mixed with traffic.

Ideally stream_int_update() should not touch these flags, but changing
this would require a very careful auditing of all users. Instead here
what we do is that we respect the flag if the channel still has output
data. This way the flag will automatically disappear once the buffer is
empty, and the applet function will be called only when input data
remains, if at all.

This patch alone is not enough to observe the behaviour change on the
stats page because another bug takes over, addressed by next patch
(BUG/MEDIUM: stats: don't ask for more data as long as we're responding).
When both are applied, dumping stats for 5k backends over a 10 Mbps link
take 1% CPU instead of 100%, with 1.5k epoll_wait() calls instead of 80k.

This fix should be backported at least as far as 1.5.

src/stream_interface.c

@@ -780,7 +780,7 @@ void stream_int_update(struct stream_interface *si)
 				ic->rex = TICK_ETERNITY;
 			}
 		}
-		else {
+		else if (!(si->flags & SI_FL_WAIT_ROOM) || !co_data(ic)) {
 			/* (re)start reading and update timeout. Note: we don't recompute the timeout
 			 * everytime we get here, otherwise it would risk never to expire. We only
 			 * update it if is was not yet set. The stream socket handler will already