use leader read when tryFollower is fallback from accessKnownLeader

[you06]

Recently, we made some changes to the selector states, the tryFollower is not only used as a fallback state of accessKnownLeader, this PR keeps the compatibility of tryFollower's requests.

Before the changes we made:

accessKnownLeader -> tryFollower -> switch leader when success, and the flag KVContext.ReplicaRead is always false here.

Now there is another path lead to tryFollower state:

accessFollower -> tryFollower, in this case, KVContext.ReplicaRead is set to true in tryFollower state.

To make the behavior compatible with earlier versions, we need to set a flag for the accessKnownLeader -> tryFollower path, and that's fromKnownLeader. It's renamed from fromOnNotLeader for better semantic.

cc @crazycs520

[crazycs520]

@cfzjywxk @zyguan @ekexium PTAL