multi: split database storage into remote and local instances

In this commit, we split the database storage into two classes: remote
and local data. If etcd isn't active, then everything is actually just
local though we use two pointers everywhere. If etcd is active, then
everything but the graph goes into the remote database.

peer/brontide.go

@@ -456,7 +456,7 @@ func (p *Brontide) loadActiveChannels(chans []*channeldb.OpenChannel) (
 		// Before we register this new link with the HTLC Switch, we'll
 		// need to fetch its current link-layer forwarding policy from
 		// the database.
-		graph := p.cfg.ChannelDB.ChannelGraph()
+		graph := p.cfg.ChannelGraph
 		info, p1, p2, err := graph.FetchChannelEdgesByOutpoint(chanPoint)
 		if err != nil && err != channeldb.ErrEdgeNotFound {
 			return nil, err