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/addons/promex/service-prometheus.c b/addons/promex/service-prometheus.c
index 5622b80..4fee8c3 100644
--- a/addons/promex/service-prometheus.c
+++ b/addons/promex/service-prometheus.c
@@ -1598,7 +1598,7 @@
 
 		case PROMEX_ST_END:
 			if (!(res->flags & CF_SHUTR)) {
-				res->flags |= CF_READ_NULL;
+				res->flags |= CF_READ_EVENT;
 				sc_shutr(sc);
 			}
 	}
@@ -1614,7 +1614,7 @@
 	return;
 
   error:
-	res->flags |= CF_READ_NULL;
+	res->flags |= CF_READ_EVENT;
 	sc_shutr(sc);
 	sc_shutw(sc);
 }