Use cast instead of call to enqueue

[rhnonose]

When there's an increased amount of workers enqueing work on the feeder, using a GenServer.call is not realistic, even with a really high timeout, and a response is not strictly required. This PR changes call to cast for stop, resume, pause and enqueue.

[coveralls]

Coverage decreased (-0.07%) to 98.148% when pulling 1940c31 on rhnonose:master into b635913 on fredwu:master.

[fredwu]

Thanks @rhnonose! Is this related to fredwu/crawler#11?

[rhnonose]

@fredwu It's unrelated.

[fredwu]

Hmm, even if you set the timeout to infinity? The reason for using call was if an enqueue'd job crashes the queue should know about it. 🤔

[rhnonose]

I think that's gonna hang all the processes until it exhausts memory, at least the way it's used in the crawler.

Since the crawler AFAIK runs a breadth-first on the links, the processes waiting will increase exponentially (speculating).

But thinking more for opq itself as a stand-alone lib, I'm not sure. call with timeout to infinity will only make it a tighter bottleneck IMO.

[fredwu]

Merged, thanks @rhnonose!