lnd+graph: add GraphSource interface and implementation

This commit adds a new GraphSources interface that LND requires for
graph related read-only queries. As of this commit, the interface is
empty but it will be populated over the next couple of commits. We add
an implementation of this interface backed by a pointer to a
graphdb.ChannelGraph.

The infrustructure is put into place so that the GraphSoure provided to
LND can be overridden by a caller of the lnd.Main function. By default,
LND will satisfy the interface itself via the new `graphsource.DBSource`
struct.

server.go

@@ -45,6 +45,7 @@ import (
 	graphdb "github.com/lightningnetwork/lnd/graph/db"
 	"github.com/lightningnetwork/lnd/graph/db/models"
 	graphsession "github.com/lightningnetwork/lnd/graph/session"
+	"github.com/lightningnetwork/lnd/graph/sources"
 	"github.com/lightningnetwork/lnd/healthcheck"
 	"github.com/lightningnetwork/lnd/htlcswitch"
 	"github.com/lightningnetwork/lnd/htlcswitch/hop"
@@ -261,6 +262,10 @@ type server struct {
 
 	graphDB *graphdb.ChannelGraph
 
+	// graphSource can be used for any read only graph queries. This may be
+	// implemented by this LND node or some other external source.
+	graphSource sources.GraphSource
+
 	chanStateDB *channeldb.ChannelStateDB
 
 	addrSource channeldb.AddrSource
@@ -507,7 +512,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 	chansToRestore walletunlocker.ChannelsToRecover,
 	chanPredicate chanacceptor.ChannelAcceptor,
 	torController *tor.Controller, tlsManager *TLSManager,
-	leaderElector cluster.LeaderElector,
+	leaderElector cluster.LeaderElector, graphSource sources.GraphSource,
 	implCfg *ImplementationCfg) (*server, error) {
 
 	var (
@@ -613,6 +618,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 		cfg:            cfg,
 		implCfg:        implCfg,
 		graphDB:        dbs.GraphDB,
+		graphSource:    graphSource,
 		chanStateDB:    dbs.ChanStateDB.ChannelStateDB(),
 		addrSource:     addrSource,
 		miscDB:         dbs.ChanStateDB,