REORG: channel: Rename CF_READ_NULL to CF_READ_EVENT

CF_READ_NULL flag is not really useful and used. It is a transient event
used to wakeup the stream. As we will see, all read events on a channel may
be resumed to only one and are all used to wake up the stream.

In this patch, we introduce CF_READ_EVENT flag as a replacement to
CF_READ_NULL. There is no breaking change for now, it is just a
rename. Gradually, other read events will be merged with this one.
diff --git a/src/ssl_sock.c b/src/ssl_sock.c
index 5bdab8c..0fea7dd 100644
--- a/src/ssl_sock.c
+++ b/src/ssl_sock.c
@@ -7344,7 +7344,7 @@
 	if (conn) {
 		if (conn->flags & (CO_FL_EARLY_SSL_HS | CO_FL_SSL_WAIT_HS)) {
 			sc_ep_set(s->scf, SE_FL_WAIT_FOR_HS);
-			s->req.flags |= CF_READ_NULL;
+			s->req.flags |= CF_READ_EVENT;
 			return ACT_RET_YIELD;
 		}
 	}