chanfitness: Add channel event log structure

This commit adds a chanfitness package which will be used to track
channel health and performance metrics. It adds a channel event
structure which will be used to track channel opens/closes and peer
uptime.

The eventLog implements an uptime function which calcualtes uptime
over a given period and a lifespan function which returns the time
when the log began monitoring the channel and, if the channel is
closed, the time when it stopped moitoring it.

log.go

@@ -11,6 +11,7 @@ import (
 	"github.com/lightningnetwork/lnd/build"
 	"github.com/lightningnetwork/lnd/chainntnfs"
 	"github.com/lightningnetwork/lnd/chanbackup"
+	"github.com/lightningnetwork/lnd/chanfitness"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/channelnotifier"
 	"github.com/lightningnetwork/lnd/contractcourt"
@@ -98,6 +99,7 @@ func init() {
 
 	addSubLogger(routerrpc.Subsystem, routerrpc.UseLogger)
 	addSubLogger(wtclientrpc.Subsystem, wtclientrpc.UseLogger)
+	addSubLogger(chanfitness.Subsystem, chanfitness.UseLogger)
 }
 
 // addSubLogger is a helper method to conveniently create and register the