Fix race conditions in pubsub

[arturaz]

So... remember this: io.lettuce.core.RedisCommandTimeoutException: Command timed out after 1 minute(s) ?

After building lettuce with a patch to include command name in the timeout message it turned out that the timeouting command is SUBSCRIBE, which was weird. Until I looked at the code that does the subscription.

These seems to be buggy:

redis4cats/modules/streams/src/main/scala/dev/profunktor/redis4cats/pubsub/internals/Subscriber.scala

Lines 35 to 42 in 63532d6

	override def subscribe(channel: RedisChannel[K]): Stream[F, V] =
	Stream
	.resource(Resource.eval(state.get) >>= PubSubInternals.channel[F, K, V](state, subConnection).apply(channel))
	.evalTap(_ =>
	FutureLift[F]
	.lift(subConnection.async().subscribe(channel.underlying))
	)
	.flatMap(_.subscribe(500).unNone)

redis4cats/modules/streams/src/main/scala/dev/profunktor/redis4cats/pubsub/internals/Subscriber.scala

Lines 53 to 62 in 63532d6

	override def psubscribe(
	pattern: RedisPattern[K]
	): Stream[F, RedisPatternEvent[K, V]] =
	Stream
	.resource(Resource.eval(state.get) >>= PubSubInternals.pattern[F, K, V](state, subConnection).apply(pattern))
	.evalTap(_ =>
	FutureLift[F]
	.lift(subConnection.async().psubscribe(pattern.underlying))
	)
	.flatMap(_.subscribe(500).unNone)

• You can't do state.get from a Ref and expect that state to hold while you go on to perform side-effects.
• Usage of .unNone is highly questionable, it just filters out None's, I guess this was intended to be unNoneTerminate.
• The logic was generally hard to follow. There is one Topic shared between multiple streams, but each subscribe gets it's own redis listener and Dispatcher? Why?

So I rewrote the logic to:

• Maintain separate AtomicCell state maps for channels and patterns.
• Create a new subscription, topic and Dispatcher first time someone subscribes to channel or pattern.
• In case someone subscribes to the same thing again the subscriber count is increased.
• Subscriber count is decreased when the Stream is terminated.
• unsubscribe finishes all Streams.
• Cleanup is performed when the last stream terminates.

I also changed the return type of publish to match the type from Lettuce.

@yisraelU I would appreciate you double-checking the logic here.