Always honor return value in BP process_events_async

[tnull]

The current default for should_break is true, which is fine, except that we don't override it in the first case of the select, i.e., when ChannelManager::get_persistable_update_future is already available:

rust-lightning/lightning-background-processor/src/lib.rs

Lines 469 to 479 in a9534fe

	define_run_body!(persister,
	chain_monitor, chain_monitor.process_pending_events_async(async_event_handler).await,
	channel_manager, channel_manager.process_pending_events_async(async_event_handler).await,
	gossip_sync, peer_manager, logger, scorer, should_break, {
	select_biased! {
	_ = channel_manager.get_persistable_update_future().fuse() => true,
	exit = sleeper(Duration::from_millis(100)).fuse() => {
	should_break = exit;
	false
	}
	}