[FEATURE REQUEST] pause server

[dcboy]

Is your feature request related to a problem? Please describe.
I think I need to suspend a certain server node to receive and process tasks

Describe the solution you'd like
asynq server pause
asynq server unpause

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
When a certain node needs to be updated, I cannot directly kill the process. I must first suspend receiving tasks and wait for all current tasks to be executed before I can update.