graph+server: add template for new graph Builder sub-system

This is preparation for an upcoming commit that will move over various
responsibilities from the ChannelRouter to the graph Builder. So that
that commit can be a pure code-move commit, the template for the new
sub-system is added up front here.

graph/builder.go

@@ -0,0 +1,57 @@
+package graph
+
+import (
+	"sync"
+	"sync/atomic"
+)
+
+// Config holds the configuration required by the Builder.
+type Config struct{}
+
+// Builder builds and maintains a view of the Lightning Network graph.
+type Builder struct {
+	started atomic.Bool
+	stopped atomic.Bool
+
+	cfg *Config
+
+	quit chan struct{}
+	wg   sync.WaitGroup
+}
+
+// NewBuilder constructs a new Builder.
+func NewBuilder(cfg *Config) (*Builder, error) {
+	return &Builder{
+		cfg:  cfg,
+		quit: make(chan struct{}),
+	}, nil
+}
+
+// Start launches all the goroutines the Builder requires to carry out its
+// duties. If the builder has already been started, then this method is a noop.
+func (b *Builder) Start() error {
+	if !b.started.CompareAndSwap(false, true) {
+		return nil
+	}
+
+	log.Info("Builder starting")
+
+	return nil
+}
+
+// Stop signals to the Builder that it should halt all routines. This method
+// will *block* until all goroutines have excited. If the builder has already
+// stopped then this method will return immediately.
+func (b *Builder) Stop() error {
+	if !b.stopped.CompareAndSwap(false, true) {
+		return nil
+	}
+
+	log.Info("Builder shutting down...")
+	defer log.Debug("Builder shutdown complete")
+
+	close(b.quit)
+	b.wg.Wait()
+
+	return nil
+}