Add comments and tests for BasicBuffer

I'll add the notes below:
This is because `1.XXXXX` is misunderstood like chunk key of `resume_fix.*.log`.
This may be a kind of bug, but we cannot decide whether `1.` is a part of chunk key or not, because current version of buffer plugin uses '.'(dot) as a one of chars for chunk encoding.
I think that this is a mistake of design, but we cannot fix it because updated plugin become not to be able to resume existing file buffer chunk.
We will fix it in next API version of buffer plugin.