chore: simplify fn prune_messages_to_be_saved

[quake]

The current implementation of prune_messages_to_be_saved is complex and unintuitive, it can be implemented in a much simpler way.

[contrun]

This seems to use the Ord implementation of BroadcastMessageWithTimestamp, which will sort messages according to their timestamp. It is possible that the timestamp of ChannelUpdate is smaller than that of Channel announcement (we didn't have code to specifically make the timestamp of ChannelUpdate later than the timestamp of the funding transaction block, we just use the system time).

[quake]

will order by message_id first:

fiber/src/fiber/types.rs

Lines 2435 to 2441 in 1a2ff8f

	impl Ord for BroadcastMessageWithTimestamp {
	fn cmp(&self, other: &Self) -> Ordering {
	self.message_id()
	.cmp(&other.message_id())
	.then(self.timestamp().cmp(&other.timestamp()))
	}
	}

[contrun]

Sorry. I didn't look carefully. In that case, this PR LGTM. I created a few tests for the dependency of channel gossip messages in #418 . Can you take a look and check if the code change here does not break the tests (I believe it doesn't. Still, it is better to
have some tests)?

[quake]

rebased and pushed