autopilot: rename to Capacity to Balance, define ChannelInfo()

We rename the field Capacity on local channels to Balance, define a new
method ChannelInfo on the manager config that let us query the database
for our latest channels state. Using this we will use the current local
balance instead of the channel capacity when doing allocation
calculations.

autopilot/agent_constraints.go

@@ -100,7 +100,7 @@ func (h *agentConstraints) ChannelBudget(channels []LocalChannel,
 	// present within the set of active channels.
 	var totalChanAllocation btcutil.Amount
 	for _, channel := range channels {
-		totalChanAllocation += channel.Capacity
+		totalChanAllocation += channel.Balance
 	}
 
 	// With this value known, we'll now compute the total amount of fund