feat: replica selector

[proost]

discussion: #688

When user set "SendToReplicas" only, set ReaderNodeSelector to replicaOnlySelector. Because It is same selecting replicas as reader node.

Currently, only adding reader node selector. AZ affinity or Low latency load balancing adding later.

[rueian]

I feel that we should not hide SendToReplicas when setting ReaderNodeSelector. That looks too implicit. Actually, I think we should ask users to provide SendToReplicas when ReaderNodeSelector is set:

Instead of simply (ReaderNodeSelector alone is hard for me to reason what it actually does):

	client, err := rueidis.NewClient(rueidis.ClientOption{
		ReaderNodeSelector: func(slot uint16, replicas []rueidis.ReplicaInfo) int {
			return rand.IntN(len(replicas))
		},
	})

I prefer making it more explicitly state:

	client, err := rueidis.NewClient(rueidis.ClientOption{
		SendToReplicas: func(cmd rueidis.Completed) bool {
			return cmd.IsReadOnly()
		},
		ReplicaSelector: func(slot uint16, replicas []rueidis.ReplicaInfo) int {
			return rand.IntN(len(replicas))
		},
	})

I believe the latter provides users with a better sense of control over which replicas to execute which commands.

Also, I think changing the name of ReaderNodeSelector to ReplicaSelector is more consistent with the existing naming.

[proost]

Ok, Using SendToReplicas and ReplicaSelector both makes sense to me. Even though user can select primary using ReplicaSelector, It is configured explicitly, so it is clear to me.
90387d2

[rueian]

Can we avoid this allocation by replacing g.nodes from []string to []ReplicaInfo?

[proost]

I already consider it. But i didn't. Because

1. g.nodes has primary node too. so ReplicaInfo slice type is incorrect. It can be misleading to contributors.
2. user can change value each ReplicaInfo. Changing ReplicaInfo is not problem to rueidis. but there can be side effect in the user side.

Because of duplicated allocation, i also considering reusing ReplicaInfo slice. But problem is If user holds ReplicaInfo slice in the ReplicaSelector it can be problem.

I prefer reusing ReplicaInfo slice if should reduce allocation. And in this case, we should add comment that user must not store reference ReplicaInfo slice.

[rueian]

Hi @proost,

Great considerations!

1. If naming is the concern, we can use some sort of the type alias. We can only expose ReplicaInfo to users but use another name internally.
2. We can add comments on the selector function to tell the user should not change values in the slice.

[proost]

50fad71

OK, I changed it.

[rueian]

LGTM. Thanks @proost!