Remove LogReader from public API

[lni]

LogReader will be removed from the v4 public API.

If you need to somehow get repeated or random access to some log entries, you can always sequentially record all those log entries whenever your Update method is called. You can then have full control of the copy you maintain.

Dragonboat's internal log is private, not exposing it is the best way to prevent surprises like #315, this also gives dragonboat more flexibility on when to do the compaction.