[FEATURE REQUEST] Make handler context aware

[hibiken]

We should support timeout per task and allow user to cancel task processing using the CLI.

Cancellation would be useful in two cases:

• User manually cancels a in-progress task using the CLI
• Background worker process is shutting down, so in-progress tasks should be cancelled

Optional timeout per task is used in cases such as:

• For tasks that are best-effort. If processing could not complete within a certain timeout, bail and retry later.

In order to support both cancel and timeout, we need to change the Handler interface.

type Handler interface {
    func ProcessTask(ctx context.Context, t *Task) error
}

and add Timeout function to create a timeout option when scheduling a task.

client.Schedule(task, time.Now().Add(time.Hour), asynq.Timeout(10 * time.Second))

CLI will have a new command to cancel a in-progress task:

asynqmon cancel [task_id]

A cancelled task should be moved to retry state consuming a retry count.