Add Task DB implementation using a local BoltDB.

build_scheduler/go/db/modified_tasks.go

Index: build_scheduler/go/db/modified_tasks.go
diff --git a/build_scheduler/go/db/modified_tasks.go b/build_scheduler/go/db/modified_tasks.go
index 7408481e591ff6da14a8d8e441ed3bfba459268a..0fde3e4d67996991dce98fe0ec7580b310d55ee7 100644
--- a/build_scheduler/go/db/modified_tasks.go
+++ b/build_scheduler/go/db/modified_tasks.go
@@ -61,12 +61,19 @@ func (m *ModifiedTasks) clearExpiredSubscribers() {
 // TrackModifiedTask indicates the given Task should be returned from the next
 // call to GetModifiedTasks from each subscriber.
 func (m *ModifiedTasks) TrackModifiedTask(t *Task) {
+	m.TrackModifiedTasks([]*Task{t})
+}
+
+// TrackModifiedTasks calls TrackModifiedTask on each item.
+func (m *ModifiedTasks) TrackModifiedTasks(tasks []*Task) {
 	m.mtx.Lock()
 	defer m.mtx.Unlock()
-	// Make a single copy, since GetModifiedTasks also copies.
-	t = t.Copy()
-	for _, modTasks := range m.tasks {
-		modTasks[t.Id] = t
+	for _, t := range tasks {
+		// Make a single copy, since GetModifiedTasks also copies.
+		t = t.Copy()
+		for _, modTasks := range m.tasks {
+			modTasks[t.Id] = t
+		}
 	}
 }