Different behavior on the 2 write(del and set) commands with the PreferReplica flags.

[czd890]

// it throws RedisCommandException: Command cannot be issued to a replica.
KeyDeleteAsync("a", CommandFlags.PreferReplica);

// successfully.
StringSetAsync("a", "a", flags: CommandFlags.PreferReplica);

The difference comes from the different timing of calling SelectServer.
del will use Select(command, key, flags) to select a server before building the Message (determines whether to use del or unlink depending on the server version).
And the set command will select a server in PrepareToPushMessageToBridge based on message.flags (determined by IsPrimaryOnly()).

we should use Select(command, key, DemandMaster) to fix for del and other commands(like pexpire,expire) or throw ExceptionFactory.PrimaryOnly(...)?