MINOR: connection: provide a generic data layer wakeup callback

Instead of calling conn_notify_si() from the connection handler, we
now call data->wake(), which will allow us to use a different callback
with health checks.

Note that we still rely on a flag in order to decide whether or not
to call this function. The reason is that with embryonic sessions,
the callback is already initialized to si_conn_cb without the flag,
and we can't call the SI notify function in the leave path before
the stream interface is initialized.

This issue should be addressed by involving a different data_cb for
embryonic sessions and for stream interfaces, that would be changed
during session_complete() for the final data_cb.
diff --git a/src/connection.c b/src/connection.c
index 07225e9..007d3b1 100644
--- a/src/connection.c
+++ b/src/connection.c
@@ -110,8 +110,8 @@
 		return 0;
 	}
 
-	if (conn->flags & CO_FL_NOTIFY_SI)
-		conn_notify_si(conn);
+	if (conn->flags & CO_FL_WAKE_DATA)
+		conn->data->wake(conn);
 
 	/* Last check, verify if the connection just established */
 	if (unlikely(!(conn->flags & (CO_FL_WAIT_L4_CONN | CO_FL_WAIT_L6_CONN | CO_FL_CONNECTED))))