autopilot/prefattach+interface: add API NodeScores

This commit adds a new method NodeScores to the AttachementHeuristic
interface. Its intended use is to score a set of nodes according to
their preference as channel counterparties.

The PrefAttach heuristic gets a NodeScores method that will score the
ndoes according to their number of already existing channels, similar to
what is done already in Select.

autopilot/interface.go

@@ -98,6 +98,10 @@ type AttachmentDirective struct {
 	// Addrs is a list of addresses that the target peer may be reachable
 	// at.
 	Addrs []net.Addr
+
+	// Score is the score given by the heuristic for opening a channel of
+	// the given size to this node.
+	Score float64
 }
 
 // AttachmentHeuristic is one of the primary interfaces within this package.
@@ -127,6 +131,24 @@ type AttachmentHeuristic interface {
 	Select(self *btcec.PublicKey, graph ChannelGraph,
 		amtToUse btcutil.Amount, numNewChans uint32,
 		skipNodes map[NodeID]struct{}) ([]AttachmentDirective, error)
+
+	// NodeScores is a method that given the current channel graph, current
+	// set of local channels and funds available, scores the given nodes
+	// according to the preference of opening a channel with them. The
+	// returned channel candidates maps the NodeID to an attachemnt
+	// directive containing a score and a channel size.
+	//
+	// The scores will be in the range [0, M], where 0 indicates no
+	// improvement in connectivity if a channel is opened to this node,
+	// while M is the maximum possible improvement in connectivity. The
+	// size of M is up to the implementation of this interface, so scores
+	// must be normalized if compared against other implementations.
+	//
+	// NOTE: A NodeID not found in the returned map is implicitly given a
+	// score of 0.
+	NodeScores(g ChannelGraph, chans []Channel,
+		fundsAvailable btcutil.Amount, nodes map[NodeID]struct{}) (
+		map[NodeID]*AttachmentDirective, error)
 }
 
 // ChannelController is a simple interface that allows an auto-pilot agent to