chanfitness: set open time for channels with offline peers

This commit addresses a bug in the channel event store
where the opened time of a channel event log was not
set for peers that were offline on startup.

Previously, opened time was set to the time of the first
event in the event log. This worked for online peers,
because the eventlog was created with an initial online
event. However, offline peers had no inital event so had
no open time set.

This commit simplifies the creation of an event log by
removing the initial event and setting open time for all
event logs. This has the effect of potentially introducing
a gap between opened time for a log and the first peer
online event for peers with channels that exist at startup
if a peer takes time to reconnect. However, the cost of this
is less than the benefit of reducing the bug-prone custom
code path that was previously in place.

chanfitness/chaneventstore.go

@@ -207,15 +207,9 @@ func (c *ChannelEventStore) addChannel(channelPoint wire.OutPoint,
 		return
 	}
 
+	// Create an event log for the channel.
 	eventLog := newEventLog(channelPoint, peer, time.Now)
 
-	// If the peer is online, add a peer online event to indicate its starting
-	// state.
-	online := c.peers[peer]
-	if online {
-		eventLog.add(peerOnlineEvent)
-	}
-
 	c.channels[channelPoint] = eventLog
 }