conc: disabled task runner by default; (#1113)

Make the task runner opt-in. This change also makes the runner a background module, so it doesn't prevent application shutdown if there are no tasks left.

pkg/conc/task_runner/task_runner.go

@@ -18,17 +18,20 @@ import (
 //
 // Try to avoid using this module if possible. It is only meant as a last resort.
 //
+// Enable it by setting task_runner.enabled = true
+//
 //go:generate mockery --name TaskRunner
 type TaskRunner interface {
 	kernel.Module
 	RunTask(task kernel.Module) error
 }
 
 type Settings struct {
-	Enabled bool `cfg:"enabled" default:"true"`
+	Enabled bool `cfg:"enabled" default:"false"`
 }
 
 type taskRunner struct {
+	kernel.BackgroundModule
 	lck          sync.Mutex
 	done         bool
 	pendingTasks chan kernel.Module