refactor: move various duties from ChannelRouter to graph.Builder

This commit is a large refactor that moves over various responsibilities
from the ChannelRouter to the graph.Builder. These include all graph
related tasks such as:
- graph pruning
- validation of new network updates & persisting new updates
- notifying topology update clients of any changes.

This is a large commit but:
- many of the files are purely moved from `routing` to `graph`
- the business logic put in the graph Builder is copied exactly as is
  from the ChannelRouter with one exception:
- The ChannelRouter just needs to be able to call the Builder's
  `ApplyChannelUpdate` method. So this is now exported and provided to
the ChannelRouter as a config option.
- The trickiest part was just moving over the test code since quite a
  bit had to be duplicated.

graph/stats.go

@@ -0,0 +1,69 @@
+package graph
+
+import (
+	"fmt"
+	"sync"
+	"time"
+)
+
+// routerStats is a struct that tracks various updates to the graph and
+// facilitates aggregate logging of the statistics.
+type routerStats struct {
+	numChannels uint32
+	numUpdates  uint32
+	numNodes    uint32
+	lastReset   time.Time
+
+	mu sync.RWMutex
+}
+
+// incNumEdges increments the number of discovered edges.
+func (g *routerStats) incNumEdgesDiscovered() {
+	g.mu.Lock()
+	g.numChannels++
+	g.mu.Unlock()
+}
+
+// incNumUpdates increments the number of channel updates processed.
+func (g *routerStats) incNumChannelUpdates() {
+	g.mu.Lock()
+	g.numUpdates++
+	g.mu.Unlock()
+}
+
+// incNumNodeUpdates increments the number of node updates processed.
+func (g *routerStats) incNumNodeUpdates() {
+	g.mu.Lock()
+	g.numNodes++
+	g.mu.Unlock()
+}
+
+// Empty returns true if all stats are zero.
+func (g *routerStats) Empty() bool {
+	g.mu.RLock()
+	isEmpty := g.numChannels == 0 &&
+		g.numUpdates == 0 &&
+		g.numNodes == 0
+	g.mu.RUnlock()
+	return isEmpty
+}
+
+// Reset clears any router stats and sets the lastReset field to now.
+func (g *routerStats) Reset() {
+	g.mu.Lock()
+	g.numChannels = 0
+	g.numUpdates = 0
+	g.numNodes = 0
+	g.lastReset = time.Now()
+	g.mu.Unlock()
+}
+
+// String returns a human-readable description of the router stats.
+func (g *routerStats) String() string {
+	g.mu.RLock()
+	str := fmt.Sprintf("Processed channels=%d updates=%d nodes=%d in "+
+		"last %v", g.numChannels, g.numUpdates, g.numNodes,
+		time.Since(g.lastReset))
+	g.mu.RUnlock()
+	return str
+}