Loader: add commands for pausing SQS pulling

[chuwy]

If Redshift is faulty state it would make sense to stop receiving messages. Otherwise we need to resend many messages that were acked, but failed being loaded.

[benjben]

Is your idea to make the loader able to deal with signals to never stop it ?

[chuwy]

Yes, but after writing this I realized that it's impossible to resume it with a message because our queue is FIFO, so if you sent a stop message, then few others shredding_complete then resume - the loader will never get the resume because it has to process shredding_complete first.

So unfortunately the only thing we can add is to pause for X minutes, which is not that useful.

[benjben]

Ah ok, I was talking about Linux signals. Indeed that's not possible to use SQS for this.