Free the holding cells during background timer ticks

We currently free the channel holding cells in
`get_and_clear_pending_msg_events`, blocking outbound messages
while we do so. This is fine, but may block the message pipeline
longer than we need to. In the next commit we'll push
timer-originating channel fee updates out through the holding cell
pipeline, leaning more on that freeing in the future.

Thus, to avoid a regression in message time, here we clear the
holding cell after processing all timer events. This also avoids
needing to change tests in the next commit.

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -3678,6 +3678,13 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 
 			self.remove_stale_resolved_payments();
 
+			// Technically we don't need to do this here, but if we have holding cell entries in a
+			// channel that need freeing, it's better to do that here and block a background task
+			// rather than blocking the message queueing pipeline.
+			if self.check_free_holding_cells() {
+				should_persist = NotifyOption::DoPersist;
+			}
+
 			should_persist
 		});
 	}