REORG: channel: Rename CF_WRITE_NULL to CF_WRITE_EVENT

As for CF_READ_NULL, it appears CF_WRITE_NULL and other write events on a
channel are mainly used to wake up the stream and may be replace by on write
event.

In this patch, we introduce CF_WRITE_EVENT flag as a replacement to
CF_WRITE_EVENT_NULL. There is no breaking change for now, it is just a
rename. Gradually, other write events will be merged with this one.
diff --git a/src/backend.c b/src/backend.c
index bac8a08..6e7c989 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1836,7 +1836,7 @@
 	if (!sc_state_in(s->scb->state, SC_SB_EST|SC_SB_DIS|SC_SB_CLO) &&
 	    (srv_conn->flags & CO_FL_WAIT_XPRT) == 0) {
 		s->conn_exp = TICK_ETERNITY;
-		sc_oc(s->scb)->flags |= CF_WRITE_NULL;
+		sc_oc(s->scb)->flags |= CF_WRITE_EVENT;
 		if (s->scb->state == SC_ST_CON)
 			s->scb->state = SC_ST_RDY;
 	}