MINOR: filters: Add check_timeouts callback to handle timers expiration on streams

A filter can now be notified when a stream is woken up because of an expired
timer.

The documentation and the TRACE filter have been updated.
diff --git a/src/filters.c b/src/filters.c
index 8c3485e..91404a4 100644
--- a/src/filters.c
+++ b/src/filters.c
@@ -398,6 +398,21 @@
 }
 
 /*
+ * Calls 'check_timeouts' for all filters attached to a stream. This happens when
+ * the stream is woken up because of expired timer.
+ */
+void
+flt_stream_check_timeouts(struct stream *s)
+{
+	struct filter *filter;
+
+	list_for_each_entry(filter, &strm_flt(s)->filters, list) {
+		if (FLT_OPS(filter)->check_timeouts)
+			FLT_OPS(filter)->check_timeouts(s, filter);
+	}
+}
+
+/*
  * Called when a backend is set for a stream. If the frontend and the backend
  * are not the same, this function attaches all backend filters to the
  * stream. Returns -1 if an error occurs, 0 otherwise.