refactor(epaxos.go) add comments for updateAttr and conflicts map

@imoraru
I just begin to go through the codebase. Add some comments.
One question:
Is there any documentation about the checkpoint stuff? Is that only for clearing the dep map?
Can we just update the map after executing a command, like
``` go
id, ok := dep[c.Key]
if ok && id == c.Id {
    delete(dep, c.Key)
}
```