Redis pubsub fixes

[yazd]

This fixes two issues in redis pubsub implementation.

• The first issue (solved in 1st commit) is an infinite loop (that causes 100% CPU) that happens when the redis server exits while listening to pubsub messages.
  The following code shows a reproduction case:

import vibe.db.redis.redis;
import vibe.core.core;
import vibe.core.log;

void main() {
	runTask(&listenRedis);
	runApplication();
}

void listenRedis() {
	auto redis = connectRedis("127.0.0.1");
	auto subs = redis.createSubscriber();
	subs.subscribe("test");

	logInfo("subscribed");

	auto task = subs.listen((string channel, string message) {
		logInfo("received message on %s: %s", channel, message);
	});
	task.join();

	logInfo("done listening");
}

Redis server must be stopped to face the issue.

• Second issue (solved in 2nd commit) is that the task doesn't join back or terminate because the code in teardown was waiting for Action.STOP message, but that will never arrive, because it was already consumed by the handler. I don't think there are cases when that is not the case.

[s-ludwig]

Although it is a bit difficult to keep track of all involved code paths, the change looks good to me! I'd just modify the test case for added robustness.